Add filter by category

[MikeRalphson]

• add filtering by category logic
• add UI

[MikeRalphson]

Filtering by category is now implemented: https://apis.guru/?category=social

Just needs a menu option / select drop-down etc adding.

Also filtering by tag (which is case-sensitive at the moment): https://apis.guru/?tag=helpWanted and filtering by status (which is derived from added/updated dates): https://apis.guru/?status=new / https://apis.guru/?status=updated