Support key schemata and column nullability

Summary:
Ref T1191. The major issue motivation here is that InnoDB keys have a maximum length of 767 bytes. When we move `utf8` colums to `utf8mb4` columns, they'll jump from 3 bytes per character to 4 bytes per character, which may make some indexes too long. Add key schema to help spot this.

Also add nullability since it doesn't hurt.

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10499