Further simplify SearchAttachController

Summary: Try to break this apart a little better in preparation for D595. No
functional changes, just refactored the relatively large processRequest()
method.
Test Plan:

• Attached and detached revisions from tasks.
• Attached and detached tasks from revisions.
• Merged tasks.

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 725