Allow construction of ApplicationSearch queries with GET

Summary:
Ref T3775 (discussion here). Ref T2625.

T3775 presents two problems:

1. Existing tools which linked to /differential/active/epriestley/ (that is, put a username in the URL) can't generate search links now.
2. Humans can't edit the URL anymore, either.

I think (1) is an actual issue, and this fixes it. I think (2) is pretty fluff, and this doesn't really try to fix it, although it probably improves it.

The fix for (1) is:

• Provide a helper to read a parameter containing either a list of user PHIDs or a list of usernames, so /?users[]=PHID-USER-xyz (from a tokenizer) and /?users=alincoln,htaft (from an external program) are equivalent inputs.
• Rename all the form parameters to be more digestable (authorPHIDs -> authors). Almost all of them were in this form already anyway. This just gives us ?users=alincoln instead of userPHIDs=alincoln.
• Inside ApplicationSearch, if a request has no query associated with it but does have query parameters, build a query from the request instead of issuing the user's default query. Basically, this means that /differential/ runs the default query, while /differential/?users=x runs a custom query.

Test Plan: {F56612}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625, T3775

Differential Revision: https://secure.phabricator.com/D6840