Include resolved topics as a match in autocomplete list

[neiljp]

I noticed that when autocompleting a topic when composing a message, that the topic did not appear in the list since it was resolved.

This is also an issue in the topic compose box - eg. composing to #zulip-terminal, and entering github into the topic box and cycling through autocomplete options doesn't show any resolved topics, such as ✔ GitHub PR template (#T830).

Since a resolved topic is essentially a topic with a special character (and space) prefix, this can be manually worked-around by copying the character into the textbox.

To fix this properly we might consider both resolved and unresolved topics when matching.

The right solution to this may start from #1358, since then github would match on the second word of the resolved topic above and be accessible. However, there may be other factors to consider to enable this to be fully fixed, eg excluding characters such as the resolved character (so it is 'not a word'), or another way of treating resolved topics normally. This may be necessary since we may or may not want resolved topics at a particular priority in search order.

In any case, once #1358 is resolved, adding test cases involving resolved topics would be a useful first step to characterize the behavior.