Implement join group and leave group

Join group:

- Added to group.js
- Secondary join clip should be shown to non-members when the current user is not a member and the group is joinable
- Clicking the button should join the user to the group and either re-render or refresh the page

Leave group:

- Leave group widget triggered by action button in My Memberships
- When clicked, shows a modal containing "Are you sure you want to leave the following group(s):"
- Underneath, shows a list of the selected groups to leave, in the same way as the upload list shows.
- Groups are left one by one. The same spinner/checks progress indicators should be used when leaving is in progress.
- When you are the only manager in the group, leaving should fail. This should be caught with a cross and notification message.