[RFC] Add tagging support

This PR introduces basic tagging support.

- [x] Tags can be created and added to torrents
- [x] Tags can have properties
- [x] Torrents can have property values
- [x] Property values are displayed as infos on the torrent page
- [x] Torrents can be searched by tags
- [ ] Properties can be added to tags via GUI
- [ ] Property fields are added via Javascript/AJAX when a tag is added

I'm thinking about leaving the last two parts for another PR to get the tagging support included in the master as soon as possible.

Comments?