Issue #101: Optional membership banner for members, membership status in the header, membership info on the profile page

[lilliputten]

There are only two changes in data model: added get_membership and is_member methods to the User model.

• Issue #101: Optional registration splash banner: show only for non-members. (The page looks too empty without this banner. It will need to be filled with something.)

• Issue #101: Added indication of the membership to the page header (by star icon before the user name, instead of person icon).

• Issue #99: Show membership information on the profile page.