Generate expected and comparison schemata

Summary:
Ref T1191. This builds on the "view of the database as it exists" by building a view of the database as it is expected to exist (this is mostly empty for now) and comparing the two. We now render a view of the "comparison schema", which is the actual schema merged with the expected schema and annotated with the differences.

(I'm merging them like this because it makes it easier to handle both "missing" and "surpulus" warnings in a consistent way. If we tried to annotate just the actual or expected schema, the absence of components which are expected to exist is messy to handle.)

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10496