1783: Switch to server-side sessions r=mergify[bot] a=nextgens
## What type of PR?
bug-fix
## What does this PR do?
It simplifies session management.
- it ensures that sessions will eventually expire (*)
- it implements some mitigation against session-fixation attacks
- it switches from client-side to server-side sessions (in Redis)
It doesn't prevent us from (re)-implementing a "remember_me" type of feature if that's considered useful by some.
Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>