Deprecate use of user's slurm_user, cache_dir and slurm_accounts

[tcompa]

As part of version 2.6.0, the new ground truth for these parameters will be the values stored in the new user_settings table.

We cannot drop the user-table columns in 2.6.0, because the version update will need to also update data, as in:

1. Update fractal-server version to 2.6.0
2. Run set-db script, which applies schema migrations (and creates user-settings table)
3. Run update-db-data script, which creates settings for each user (and fills them with current values - where applicable)
4. You are now in a state where you could run a job, and it would use the new user-settings values. In this same state, the API for viewing/changing user settings is already in place, while the API for the old user attributes is already gone.
5. Update fractal-server version to 2.6.1 (note: we expect this to take place right away, during the same maintenance session)
6. Run set-db script, which removes old columns from user table (and also implements Make UserOAuth.user_settings_id required #1781)

The current issue, like #1781, will be implemented and merged for 2.6.1