PTV-1890: Add an anonymous ID to users

[MrCreosote]

Any new users will have an ID automatically assigned. Anonymous IDs will be assigned to current users when they're pulled from the DB for the first time after upgrading the service.

Next steps:

• Expose in /me and testmode and admin /user endpoints
• Add admin method to look up users by anonymous ID
• Release notes

[codecov[bot]]

Codecov Report

Merging #400 (d0fc7da) into develop (ae6f3c2) will decrease coverage by 0.09%. The diff coverage is 93.33%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #400 +/- ## ============================================= - Coverage 92.89% 92.80% -0.09% + Complexity 2141 2118 -23 ============================================= Files 125 125 Lines 7483 7465 -18 Branches 1198 1171 -27 ============================================= - Hits 6951 6928 -23 - Misses 489 494 +5 Partials 43 43 ```