Add ability to edit tags on font edit screen

Font tags would be a lot more useful if they were standard across all providers. With so many fonts, this is a lot of work. We can split up the work by allowing people to add tags to individual fonts and combine those tags in the released code. People might have reason to add tags to fonts in their local installs even if it's not used by others, so it makes sense to add an interface for that. And to get that back in to the released code, maybe a separate export module.