feat(flags): Set up initial UI-reorganization for feature management

[havenbarnes]

Problem

Initial setup for: #25938

Right now this is just a UI re-organization and does not have any functional changes on the sub-pages themselves. The next step is to customize the Experiments tab to only show relevant items, recall from RFC "The "Experiments" section will be subset of all the experiments we have that map to features"

Changes

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

Yes

How did you test this code?

• Ran locally
• Will add Cypress tests validating navigation after creating, editing items in each section

[github-actions]

Size Change: +107 B (+0.01%)

Total Size: 1.16 MB

ℹ️ View Unchanged

Filename	Size	Change
frontend/dist/toolbar.js	1.16 MB	+107 B (+0.01%)

_{compressed-size-action}

[raquelmsmith]

I think the left nav item should be "Features" and go at the top under Notebooks

I was also thinking that Features would be a list of features - eg "new friend inviter" or "document creator" - these are not features we offer in posthog, but instead a thing that someone makes to track their own feature development and rollout.

So each "feature" should have a section for "linked flags" which is the flags that someone has said are related to the feature. Same for experiments.

We can chat through this on monday if you'd like!

[havenbarnes]

note for reviewing later: these lazy imports are working well in local dev, but it is unprecedented in the app. Check to make sure there isn't some reason for not using it before. If so, it's a superfluous optimization anyway, but nice for speedy loads