Remove user 1 special behavior

Problem/Motivation

Now that https://www.drupal.org/node/2910500 → is in core, it is less expected that uid 1 is an admin.

Getting rid of special behavior for user uid 1 ensure better consistency.

Remaining tasks

Need to open a new branch: 2.0.x
Raise core requirement to 10.3
Remove code handling uid 1
Fix tests

User interface changes

Some settings form element description.