add keywords property for each file

[springraaar]

currently the website allows searching for map categories. This uses a "tags" parameter that's unrelated with the existing DB table with rapid tags.

There should be a keywords property for each file (free text).

The map categories would also be set there.

An upload/edit form could show select boxes allowing the uploader to add standard map tags to the keywords box instead of typing them directly for more consistency.

[springraaar]

fixed on #85

but instead of adding them to the upload form, some of them are set automatically by a trigger and the association is made indirectly by content name not the file itself (ex: map name instead of name+version).

This implies all files/versions of a given map will share the same categories/keywords.