Improve flask session usage

[podliashanyk]

Make sure that:

• Cookie data is cleaned up frequently enough. Especially relevant after storing event_ids in session in Table refresh with ntie #87. Running Howitz for a long time with lots og events being updated could increase cookie size up to its limit.
• Reuse cookie (remember me f.e.) where possible. So that user is not logged out for as long time as possible until manual logout.
• ...

[hmpf]

On testing another branch today I got the following error:

/../.virtualenvs/howitz/lib/python3.10/site-packages/werkzeug/sansio/response.py:261: U
serWarning: The 'session' cookie is too large: the value was 4159 bytes but the header requi
red 26 extra bytes. The final size was 4185 bytes but the limit is 4093 bytes. Browsers may 
silently ignore cookies larger than this. 
 dump_cookie(

While the sorted list of event ids and what is selected is session specific, the list of known events is not and can be cached.

[hmpf]

Given a session id, we can store sessions in the cache too, just have to remember to delete it on logout.

See https://flask-session.readthedocs.io/en/latest/usage.html#using-cachelib-as-a-session-backend