ENH: Within session-configurable ratings

[jerdra]

During the QC process users may choose to add new rating categories that they discover. However, with the current setup of inputting a YAML config file into the program this is fixed and not allowed. The following requirements should be met when implementing configurable ratings:

Back-end

1. Configuration should be encoded in a DB table. This is preferred over using Bottle.config because if we ever want to work with multiple users then we'd like to allow all users to see the updated available ratings
2. HTTP endpoint should be added to update the configuration table
3. An export functionality should be provided so that users could save their updated YAML file to re-use for other projects or to share. This can be done via another HTTP endpoint (i.e fetch the current schema)

Front-end

1. The QC drop-down selector should have an option to add or remove a rating for a given QC Component (button placed beside selector)
2. This should call the appropriate API endpoint to propagate the requested changes to the database
3. Finally this should trigger a re-fetch of the primary data
4. Provide an export config option to save the current configuration into a YAML file.

Note that when removing a rating one should appropriately handle other QC images that have already been rated using category that is to be removed.