Extracts the shared strategy list and list item into the `common` folder
instead of living in the environment accordion body file.
Also takes the disabled strategy handling that we use for
`StrategySeparator` and moves it into the file itself. It might be
something we want to decorate manually in the future, but we don't for
now, so this was the most straight-forward way to make it work.
Here's an initial first pass of replacing the strategy lists in release
plan milestones.
The existing MilestoneCard has been moved to a Legacy file to avoid
conflicts.
This PR places the strategies in a list and changes the background color
of the list items (the strategies themselves still have a white
background, however).
It also re-orders the buttons in the footer and places the
milestone-level drag handle outside the milestone card.

## For later
Changing out the strategy list item itself hasn't been done yet. I want
to see if we can re-use the existing strategy draggable item instead of
making a copy. There's some dependencies on project path params etc that
need to be worked out first, though, so I'd prefer to do get these
initial changes through first.
Adjusts styling of the env dropdown now that we have both release plans
and strategies.
Key points:
- simplifies strategy separator, removes inherent height. Also: extracts
it from the draggable component (it has no business knowing whether to
add that or not)
- Puts release plans and strategies in the same list so that it becomes:
```markdown
- Release plan
- strategy 1
- strategy 2
- (OR) Strategy A
- (OR) Strategy B
```
- Adjusts some padding around to make it line up properly
- Swaps a couple conditional renders for ternaries
Rendered:

## Still todo:
Handle cases where you have >50 strats and we show the warning etc. It's
a little trickier because of how it interacts with release plans, so I
wanna leave that for later.
I'm also unsure about how we handle spacing today. All the little items
have their own different spacing and I'm not sure it won't get out of
sync, but I'm also not sure how else to handle it. We should look at it
later.
Updates the strategy list based on the new designs and moves the current
versions of the touched components into `Legacy...` files (the vast
majority of changes are that and updating imports). The relevant changes
to the components are listed in their original files.
Flag on:

Flag off:

## Next steps
There's two items to review for improving these current comments (also
noted inline):
- Whether to aria-hide the "or" separator or not (I need to read up a
bit and think whether it makes sense to show that or not)
- Changing the list of strategies into an actual ordered list (`ol`).
That'd reflect the semantics better.
Next would be checking the other places we use strategy lists and then
updating those too. In doing so, I might find that some things need to
be updated, but I'll handle those when I get there.
There's also handling release plans.
## About the changes
Refactoring the colors for the light theme to be much easier to continue
with dark mode
This is the first step to finish dark mode
https://linear.app/unleash/project/[low][s][alpha]-dark-mode-in-unleash-admin-ui-31b407d13c4b/1
This PR uses `main-theme` as a placeholder for `dark-theme` for now due
to the new changes. Still need to set the correct values here.
---------
Co-authored-by: Nuno Góis <github@nunogois.com>