Revamp search API

Plan for new search system:

Search driver (eg. Algolia) is called upon to update its index each time a post/discussion is changed
(Each post is a separate object in the index which contains not only post content but also its discussion's title/tags/other metadata)

q = foo "bar baz" (author:toby OR author:franz) is:following

1. Parse q into an abstracted query object
. eg. https://github.com/pimcore/search-query-parser or https://github.com/netgen/query-translator
2. Loop through each Term in the query object, allow gambits to match and convert term into a Where object to be processed by search driver
3. Extensions may also populate the query object to enact permissions, eg. add a Where to restrict results to only tags that the user can see
4. Search driver builds its vendor-specific query by inspecting query object's Terms/Wheres, returns list of discussion IDs

However, this is complex and will take a while to implement, so it would be wise to keep it slated for 0.2 (as per current roadmap). In the meantime, we are doing flarum/framework#1339

Start by reading the linked flarum/framework#1339 issue and the current 0.2 roadmap, then compare the proposed parser, Term/Where, extension, and search-driver responsibilities with the existing search implementation. This issue does not identify files, tests, or an agreed implementation boundary, and completion is deferred rather than defined.