Better support for multiple redirect URIs

[KorvinSzanto]

This PR does a few things to better support multiple redirect URIs:

1. Switch from type='uri' to a basic pattern that forces the string to start with http:// or https://
2. Use text column type for redirects so that we can fit longer redirect uris than 255 chars
3. Update the form validation for save to validate the URIs individually rather than as a single string
4. Adds a migration to handle upgrade

The effect of those changes is I can have as many redirect URIs as I want.