This PR makes changes to how the project overview skeleton screen works.
Important changes:
- Add skeleton screens to missing elements, creating a more
comprehensive loading screen
- Split the page into different loading sections, so that we can load
the table when we fetch the next page without affecting the rest of the
page.
https://www.loom.com/share/e5d30dc897ac488ea80cfae11ffab646
Next steps:
* Hide bar if total is less than 25
* Add FE testing
This PR fixes a bug where the rendering in the frontend would only
render the last seen component if feature.lastSeenAt was set, the new
changes considers whether or not environments last seen at is present
and takes precedent over the legacy last seen at field.
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Adds last seen by environment to FeatureOverview
- Refactored FeatureEnvironmentSeen component for reusability
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Closes #
[1-1223](https://linear.app/unleash/issue/1-1223/toggle-overview-add-last-seen-by-environment)

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Signed-off-by: andreas-unleash <andreas@getunleash.ai>
## 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.
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Co-authored-by: Nuno Góis <github@nunogois.com>