Add a generic "edge.search" method

Summary:
Ref T12337. Ref T5873. This provides a generic "edge.search" method which feels like other "verison 3" *.search methods.

The major issues here are:

1. Edges use constants internally, which aren't great for an API.
2. A lot of edges are internal and probably not useful to query.
3. Edges don't have a real "id", so paginating them properly is challenging.

I've solved these things like this:

• Edges must opt-in to being available via Conduit by providing a human-readable key (like "mention" instead of "52"). This solvs (1) and (2).
• I faked a mostly-reasonable behavior for paginating.

Test Plan:
Ran various valid and invalid searches. Paginated a large search. Reviewed UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12337, T5873

Differential Revision: https://secure.phabricator.com/D17462