Dynamically allocated file sub-types, rather than hard-code

[hogsy]

Is your feature request related to a problem? Please describe.
N/A

Describe the solution you'd like
Right now, we've hard-coded a collection of potential sub-types to help organise things better. These will get used as the sub-categories when viewing a particular page, of which files are organised under.

Rather than doing this, what we should do is be able to provide the indexer with a collection of subtypes we want and hints it can use for categorising files under those subtypes (which can of course be overridden with additional dd meta files, but that can be investigated after).

Describe alternatives you've considered
N/A

Additional context
Sub-types form these categories.

As mentioned however, these are currently in a hard-coded list (though it's already designed so it's easy to add to this and should be relatively easy to migrate this into a flexible list the user can provide).