Commands to run/list migrations in ops.

[See #TBD for the Gallery side of these changes]

I'm putting the entire description for the fix for #900 here. Basically, the site changes are minimal:
1. Stop auto-running Migrations
2. Add a "Gateway" class to make it really easy for the ops components to construct a DbMigrator without having to fiddle with a bunch of internal gallery components.

On the Ops side, we're adding two new tasks:

## List Migrations
Lists all migrations in the assembly and marks the ones that have been applied:
![listmigrations](https://f.cloud.github.com/assets/7574/369740/4bd53120-a2de-11e2-9188-44c238e9e713.png)

## Run Migrations
Runs Up migrations, or generates SQL Script for Up or Down Migrations. Note that due to my feeling that we need to be extremely careful running Down migrations, I don't allow down migrations to be auto-run, you can only generate SQL. Screenshots:

Simple Up-Migration Scenario:
![runmigrations-whatif](https://f.cloud.github.com/assets/7574/369755/795f31cc-a2de-11e2-91d7-8515523ae4c3.png)

Generate an Up Script:
![runmigrations-sql](https://f.cloud.github.com/assets/7574/369758/889c2398-a2de-11e2-91b6-15bfd3c1a027.png)

The Script:
![runmigrations-upscript-sample](https://f.cloud.github.com/assets/7574/369762/91c7ea56-a2de-11e2-84ff-db018c7d1c39.png)

No Down-Migration for you!
![runmigrations-downwardsrequiresscript](https://f.cloud.github.com/assets/7574/369764/99ec6680-a2de-11e2-9872-dbe0ad678255.png)

Sample Down-Script:
![runmigrations-downscript](https://f.cloud.github.com/assets/7574/369766/a02a2dde-a2de-11e2-813e-a457817a8ecf.png)