Minor fixes to mode-line

Emacs complains that UTF-8 encoding doesn't exists which is plain
annoying for emacs users
documentation: add troubleshooting subsection in development section

Flask FAS extension use a secure cookie for authentication which
prevents transmitting it in non-encrypted flux.
For development purposes, it could be disabled, documentation has
been updated for the sake of fedocal development newbies.

Do not do this in production !