astakos: Invalidate user object before updating last login date

to ensure user object modifications that took place during the
login request, such as updated token expiration date, won't
get overridden.
astakos: Filter out surplus projects in usage view

do not display projects the user is not member to and no resources
are consumed from the available project quotas.
astakos: Return tuple in silent mode

Fix `check_conflicting_projects` function in order to return a (result,
message) tuple in silent mode, which is expected by
`validate_project_action` function.