Fixes an issue where the collaborator component would be smooshed
together when you have too many collaborators and too many flag tab
items.
The primary things I have done are:
1. Limit the amount of collaborators we show to 6 instead of 8. I
believe the number 8 was arbitrary, so let's go with 6 for now.
2. Instead of using a fixed gap, use a separator element that grows up
to a certain limit. I've added a `Separator` component, which is an
empty div with flex-grow. It feels like you should be able to do that
with gap too, but I can't think of how right now.
3. Don't allow collaborator component text (or avatars) to wrap. We
don't have a lot of space in this header, so let's keep it tight.
Additionally, I've added the `className` prop to the AvatarGroup
component so that it can be styled externally. I also cleaned up some
naming that was left in while I was at it.
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Fixes a bug introduced with the new tooltips where the system user was
shown as "User ID n" instead of "System". The "n" in this case is
actually the user's index number in the list of project owners
(including duplicates).
There's a few things happening:
1. Change the object for system owners: use `name` instead of
`description`. At the same time, remove the `description` property
completely because it's not used at the moment.
2. Remove the assignnment of `id: objectId(user)` to the user sent to
the User Avatar component. This was a leftover from when we split out
the AvatarGroup component, and is not something we use anymore.
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After:

Extracts the Avatar Group component into a `common` component and adds a
standard tooltip to all avatars.
Relates to linear issue 1-2606
This is a suggestion / proof of concept for how we can solve it. While I
think we can merge this as is, I'd also be happy to take any discussions
on other ways to approach it etc.
## Why are these changes made together?
Because extracting the avatar group without adding the new tooltip data
made the existing tooltip misbehave (it'd show up in the top left of the
screen, not synced to the avatar in any way).
I probably could have (and still can if you think it's prudent) split it
out such that the avatar gets a standardized tooltip first (and disable
it for the group card avatars), and split out the avatars in a
follow-up. Happy to do that if you think it's better.
## What does this mean?
It used to be that we had no consistent way of dealing with avatars and
tooltips. Some places had them, some places didn't. This change makes it
so that all avatars that we can show tooltips for will get the same
tooltip.
Previously, we had at least 4 different ways of dealing with tooltips:
- The HTML tooltip (that would be standardized with this PR) in the
project flags table

- The "title" that you'd get on your user avatar

- The group card list tooltip

- And sometimes you'd get nothing at all

with this change, we'll always show the same kind of tooltip if we can:

## What goes in the tooltip?
We use the `UserAvatar` component for a fair few different things and I
didn't want to extract separate components for all the different use
cases. Instead, I wanted to get an overview over what we use it for and
what is relevant info to show.
I found all the places we used it and tried to form an opinion.
This tooltip will work with a user's email, name, username, and id. If
there is no user (such as for empty avatars and avatars displaying only
"+n" for remaining members), we show no tooltip.
Following the example set by the group card avatars, we'll try to use
email or username (in that order) as the main bit of text. If the user
has an email or a username and also a name, the name will be used as
secondary text.
If the user does not have an email or username, but has a name, we'll
use the name as the main text.
If the user does not have an email, a username, or a name, we'll try to
show "User ID: N" if they have an id.
If they do not have a username, a name, an email, or an ID, we bail out
and show nothing.
## Why can you disable the tooltip?
In some cases, you might want to disable the tooltip because you have
more information to feed into it. An example of that is in the project
flags table, where we want to show more information in cases where the
user is 'unknown':

## Additional fixes
This PR also adds a few lines of CSS to fix a minor avatar layout bug.
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