Entity browser widget for user assignment in Logs

Problem/Motivation

This came up in the forum: https://farmos.discourse.group/t/person-identification/773/

With lots of users in farmOS it's hard to select them in the default "select" list when editing a log.

Proposed resolution

Instead of using the default "select" widget for the "Assigned to" user entity reference field in logs, we could use an Entity Browser widget, like we use for asset selection.

Question: what about the "Assign" bulk action? We haven't ported that to 2.x yet, but in 1.x it is also a simple select widget. Should we see if it's possible to use Entity Browser for that as well?

Remaining tasks

TBD

User interface changes

Change "Assigned to" field from select widget to Entity Browser widget.

API changes

None.

Data model changes

None.