Save last login date to the topoguide in db

In order to make easier/safier the search of inactive users, the last login date should be store in db (topoguide).
For the moment, the date is stored in Discourse db (last view).

Start by tracing the login handling and user persistence in the Python API, then inspect how Discourse's last-view date is currently used. Done means the last login date is stored in the topoguide database and can support identifying inactive users.

Written by the indexing model from the issue text.