The "REDIS_URL" variable is hard-coded in RQ Dashboard.

REDIS_URL was used by redis_store that is passed to Flask Session.
The url variable used by Flask-Redis can be changed by setting the
config_prefix.

1. Rename REDIS_URL to SESSION_REDIS_URL
2. Rename redis_store to session_redis_store
3. Use "SESSION_REDIS" config_prefix to initialize session_redis_store

This makes it clear that this Redis instance is used by Flask-Session.
REDIS_URL will be used for RQ.

This reverts commit 4ddde36f.

This broke the loading of fonts. Fonts directory should be excluded.

The browser caches the static files (js, css...).
Flask-CacheBust adds a hash to the URL of each static file to force
clients to redownload the files when changed.

In Flask, session data are stored in a cookie by default.
Using a server side session, there is no risk of putting too much data
in the session and transmissting a big cookie on every request
(only an id is sent in the cookie).

Redis is used as backend (without persistent storage).

- catch IntegrityError on patch
- disable autoflush (IntegrityError was raised before commit due to
autoflush)