SPIKE: Allow excluding terms from search

Currently Hypothesis search on the activity page functions only in an inclusive way, e.g. filter by "tags: ocean", or "user: dricker". There is no way to perform an exclusive type of search, i.e. filter by all "tags: **not** ocean".

Being able to specify search terms to **exclude** (such as with a "not" operator, for example) would greatly increase the functionality of search.

Start by reviewing the activity page's existing inclusive search behavior, including terms such as "tags: ocean" and "user: dricker". Define how an exclusion operator should work for terms such as "tags: not ocean" and document examples that demonstrate the intended search results.

Written by the indexing model from the issue text.