Roommate slug automatically generated from associated username
Signed-off-by: Balthazar Rouberol <brouberol@imap.cc>
The url /<category-slug> leads to a summary of all expenses in that category

The Roommate is the one linked to the request user.
The expenses are ordered by descendant date.
Signed-off-by: Balthazar Rouberol <brouberol@imap.cc>
Order expenses by descendant dates

Signed-off-by: Balthazar Rouberol <brouberol@imap.cc>
Pep8

Signed-off-by: Balthazar Rouberol <brouberol@imap.cc>