Eliminate expensive and potentially unnecessary `GET /json/users` call in /#administration page

[timabbott]

Currently, page_params sends down the realm_users data structure, which in turn is sourced from zerver/models.py:active_user_dicts_in_realm, an efficiently cached data structure containing (1) only active users and (2) no avatar URLs or bot owners, but otherwise the same data that GET /json/users returns. It seems like we could make the following changes:

• [x] (potentially) Do the active_user_ids part of #3709, to minimize perf impact of adding stuff to active_user_dicts_in_realm.
• [x] Add avatar_url to active_user_dict_fields (we kinda need to do this anyway for being able to show avatars of other users not in the message feed, e.g. via profiles you click to from the right sidebar).
• [ ] Add bot_owner to active_user_dict_fields (for bot users only, no sense blocking up the data set unnecessarily).
• [x] Either modify active_user_dicts_in_realm to include is_active (and inactive users; would want to rename it as well), or make a new version of the query for the /#administration page that only fetches inactive users (and thus would be relatively efficient in most realms) when you click into the "deactivated users" tab.
• [ ] Possibly at this point, make GET /json/users able to read from active_user_dicts_in_realm and thus the relevant cache.

On chat.zulip.org, the GET /json/users call takes about 500ms, so this could be a significant performance win if done right.

[showell]

update: We now have avatar_source and avatar_version in active_user_dict_fields.

[timabbott]

Cool, converted to checkboxes and added that :)

[HarshitOnGitHub]

Was going to open an issue for this. Realized that this should be done while working on PR #3597. Will start working on this after completing some of the remaining issues on my TODO.

[timabbott]

The active_user_dict_fields object is now called realm_user_dict_fields, and we've completed most of this. It probably would not be a ton of work to finish this.