Fixes for migrations

Includes 2 commits from #127. Any changes to that PR should be merged here.

This PR is in reference to #126. I have closed that PR and incorporated the changes into this one. The changes there were regarding a mismatch between the documentation on deployment and how things actually were; all leading to this work.

To fix this, I went back to the last commit on `schema/mysql.sql` before it was moved to the `migrations` folder and the history between the two was severed.  After making some spot changes to that, I then:

- Dumped all tables I was working with
- Imported the modified schema mentioned above
- Ran migrations without issue

From here, I exported my current schema and data and compared against the `migrations/schema.sql` that is currently in `master`.  There were no significant differences other than sizes of a few fields and the specification of `AUTO_INCREMENT` values that were not zero.  Many of the differences in sizes / types of fields should be reviewed and decided upon, but the bulk of the work is here and can probably be tweaked.

It would be nice to split all the sentry migrations out into their own Phinx migration, but that's tough without breaking current deployments of OpenCFP as far as migrations go.