Keep entity facts the person didn't touch when an entity form is saved #2633

Part of #2426 (F-FRM). Follow-up to #2619: findings from an independent review of the merged entity forms. They are fixed with #2630, in the same editor.

Findings

1. Medium: an edit clears the owner of facts owned by a disabled user. The owner list holds active users only, and matching is case-sensitive. A stored owner that isn't listed, or is stored in uppercase, falls back to "None", and the save stores owner: null without a word.
2. Medium: a boolean on an entity is always saved as an unsourced fact. An unset boolean is read as false, so a name-only edit adds an unsourced {"value": false} fact that wasn't there.
3. Low: stored times lose their seconds on any save. This is fixed by Keep the entry form's base version, stop rewriting untouched values, and say which draft the form shows #2630's "keep what the person didn't touch".
4. Low: fact details typed for an empty value are discarded without a message.
5. Low: an owner id that belongs to no user is accepted from a forged post.
6. Low: tests miss a re-save of an edit, the selected owner, last verified, and whether errors and the unsourced marker sit next to their field.

Scope

• The owner list holds every user. A disabled user shows as disabled, so a stored owner stays selected. Owner ids are compared as GUIDs, so case doesn't matter.
• A boolean that is unchecked and has no stored value is left out, as an empty field is. An unchecked box over a stored true saves false.
• Fact details given for an empty value are reported next to the field.
• An owner id that is not a user's is reported as an unknown owner.
• Tests:
  • A re-saved edit keeps the owner of a disabled user, the seconds of last verified and an untouched boolean's absence.
  • The edit form shows the selected owner and last verified.
  • Errors and the unsourced marker sit inside their field's fieldset.
  • A forged owner is refused.

Acceptance

• A name-only edit of an entity changes only the name fact, with every other fact byte for byte the same.