Store server feature level in Redux #4049
Description
(This isn't ripe to do any work on until the server side has been implemented.)
Per zulip/zulip#14618, we plan to add a "feature level" number to the server's API, which appears alongside zulip_version but can be used at a finer grain.
Once it's there, we'll want to observe it on /server_settings and /register, and maintain it in Redux. This will work just like #3839.
Then we'll want to use it instead of the version number anywhere we can -- basically, for any new server feature that's after 2.1.0. In particular:
stream_post_policy, for Support setting streams announcement-only, and other post policies #3131- as an optimization on how we use Improve support for accessing uploaded files on mobile zulip#14500 (discussion here)
- new Markdown features as discussed here
Priority because of #3131.