feat(environments): Environment switcher, behind flag

[Twixes]

Problem

This is step 5 of the environments project – #13418 (comment).

Before:

Changes

After:

Added EnvironmentSwitcher and CreateEnvironmentModal, which together allow managing all your environments from the navbar.

This required making OrganizationSerializer include field projects, serialized using the new simple ProjectBasicSerializer (the previous one – which inherited a lot of fields from TeamBasicSerializer – is now called ProjectBackwardCompatBasicSerializer).

How did you test this code?

API tests should continue to pass. Not testing the UI changes yet, as they're flagged.

[github-actions]

Size Change: +8 B (0%)

Total Size: 1.11 MB

ℹ️ View Unchanged

Filename	Size	Change
frontend/dist/toolbar.js	1.11 MB	+8 B (0%)

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