Add a basic, general-purpose export workflow for all objects with SearchEngine support

Summary:
Depends on D18918. Ref T13046. Ref T5954. Pull logs can currently be browsed in the web UI, but this isn't very powerful, especially if you have thousands of them.

Allow SearchEngine implementations to define exportable fields so that users can "Use Results > Export Data" on any query. In particular, they can use this workflow to download a file with pull logs.

In the future, this can replace the existing "Export to Excel" feature in Maniphest.

For now, we hard-code JSON as the only supported datatype and don't actually make any effort to format the data properly, but this leaves room to add more exporters (CSV, Excel) and data type awareness (integer casting, date formatting, etc) in the future.

For sufficiently large result sets, this will probably time out. At some point, I'll make this use the job queue (like bulk editing) when the export is "large" (affects more than 1K rows?).

Test Plan: Downloaded pull logs in JSON format.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13046, T5954

Differential Revision: https://secure.phabricator.com/D18919