workshops/models.py: Rename Project to Tag and make Event.tags many-to-many
workshops/models.py: Rename Project to Tag and make Event.tags many-to-many

We want to tag events for groupings beyond projects.  For example,
some students and instructors may have a particular interest in WiSE
(Women in Science and Engineering) events.  I think this covers the
Event.project use case in a more general way, so I've just renamed
'Project' to 'Tag' and 'Event.project' to 'Event.tags'.  Event.tags is
many-to-many so we can assign several tags to the same event, both our
traditional projects (e.g. SWC) and additional tags (e.g. WiSE).

pbanaszkiewicz/amy,wking/swc-amy,shapiromatron/amy,shapiromatron/amy,vahtras/amy,wking/swc-amy,vahtras/amy,shapiromatron/amy,wking/swc-amy,vahtras/amy,swcarpentry/amy,swcarpentry/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,swcarpentry/amy,wking/swc-amy