record meaningful data in last login date for external authority users

### Steps to reproduce
1. Create an account tied to an external authority (eg, eLife via ORCID).
2. Note recorded value in user.last_login_date (should match account creation timestamp).
3. Log out and log back in to the same account.
4. Note user.last_login_date value doesn't change.

### Expected behaviour
Last log in date should reflect something useful/reasonable about the last time a user took action (eg, timestamp from last time they created/edited an annotation), or perhaps be blank as the data now doesn't seem to represent the purpose of the data field.

### Browser/system information
Chrome Version 65.0.3325.162 (Official Build) (64-bit)/Mac OS 10.13.3.

### Additional details
Data values via Metabase.