Fixes for migrations

Includes 2 commits from #127. Any changes to that PR should be resolved 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.

The only other difference I noticed through scanning and diffs (diffs on sql files are stupid to look at when it's late :frowning:) was that the `users` table maintains a `url` field that was in the original `schema/mysql.sql`.  **Note to @mdwheele; a migration should be written to remove this if it is not needed.**

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.