Note that this will only hide/show *future* reblogs by a user, and does
nothing to remove/add reblogs that are already in the timeline. I don't
think that's a particularly confusing behavior, and it's a lot easier
to implement (similar to mutes, I believe).
* Add a StatusFilter class to identify visibility of statuses by accounts
* Extract StatusThreadingConcern from Status
* Clarify purpose of checking for nil account
* Move specs for account finder methods to concern spec
* Move account finder methods to concern
* Improve spec wording
* Use more explicit comparison to ensure correct return value
* Add coverage for .find_local! and .find_remote!
* Add some methods to the finder
* Use arel on matching_username method
* Avoid ternary in matching domain method
* Simplify finder methods
* Use an AccountFinder class to simplify lookup
