Joinable Groups

TODO:
Description for groups
UI for creating GroupCategories
UI for adding a group to a GroupCategory
Sorry, "these" as in accept_request() etc. The function returns failures with error messages on the checks above, I assume we'd want to return success to the client here. (It looks like the js is just reloading the page regardless right now, but presumably if there's an error we'll want to flash it or something. Not sure what we do elsewhere.)