This fixes a performance issue looking up UserProfile objects for
realms with a large number of users in the case that a UserProfile
object is not in the cache.

Thanks to @dbiollo for the suggestion!

Since we frequently do filters for only active users, these indexes
may help performance in some cases.

This fixes a performance issue joining a server with a large number of
users.

Thanks to @dbiollo for the suggestion!

This makes it possible to use DevAuthBackend when doing
performance/scalability testing on Zulip with many thousands of users.

It's unlikely that anyone testing this backend will find it valuable
to have more than 100 login buttons on the same page, and if they do,
they can always just change this limit.

Thanks to @dbiollo for the suggestion!

Just doing the database query is more readable, and has about the same
performance as before in the case where active user dicts for the
realm are in cache (and is substantially better in the rare case that
this isn't in the cache).

Thanks to @dbiollo for the perf investigation and suggestion!

This removes from our cache a moderate amount of totally useless alert
word data corresponding to users who don't have any alert words.

Thanks to @dbiollo for the suggestion!

The browser registers for events via loading the home view, not this
interface, and this functionality is available via the API-format
register route anyway.