Issue 2668 Allow collection owners/mods to add/invite works to their collections

Resolves issues:
http://code.google.com/p/otwarchive/issues/detail?id=2668
https://code.google.com/p/otwarchive/issues/detail?id=2929

Add the ability for moderators to invite works to their collections. They do so via the 'Add to Collections' button which appear on each work (for the works author, and any logged in user who maintains a collection)
I think it's possible to combine these nested ifs into:  `<% if @work && (is_author_of?(@work) || is_maintainer?) %>`