Associate TicketBook with Membership

Basic changes to associate TicketBook with Membership instead of MembershipType, and to nest TicketBooksController under MembershipsController.
Rename Membership#type to #membership_type

Rename `Membership#type` to `Membership#membership_type` as less generic.
Pre-fill association fields for new memberships

In the membership form, try to pre-fill automatically the activity period and the membership type selection fields.
Clean up
Change Membership#virtual_title
Autofocus username field in login form
Simplify some external links' text
Add a translation
Format
WIP on RegisterController