This commit allows us to type # in the UI and present autocomplete
results immediately with the following logic for the topic composer,
and reversed for the chat composer:
* Categories the user can access and has not muted sorted by `topic_count`
* Tags the user can access and has not muted sorted by `topic_count`
* Chat channels the user is a member of sorted by `messages_count`
So in effect, we allow searching for hashtags without a search term.
To do this we add a new `search_without_term` to each data source so
each one can define how it wants to handle this logic.
3fdb8ffb57