[Port to 3.x] Fix Various Issues with Form Panel Validation and Field Display - Part 4, Template Panel

[smg6511]

NOTE: This is part 4 of 6 of a revised submission of #15887. This should be reviewed after part 2 is merged (there are two small changes needed from that PR to make the locked switch appear correctly).

What does it do?

Ports enhancements to the Template form panel from PR #15146 to 3.x (including subsequent fixes by others).
Changes include:

1. Updated panel JS to make form layout consistent with the layout introduced in the current TV panel, and utilize new shared methods introduced in the main FormPanel via part 1 of this PR set (already merged).
2. Added a secondary Media Source combo for the preview image, as the new layout makes the main Media Source combo available only when Is Static is ticked. For the purposes of this PR, the new combo mirrors the original (so when one changes, the other reflects that same change) — this is, in effect, the current behavior, but it may be worthwhile to consider having a fully-independent secondary Source setting in the future.
3. A handful of minor lexicon updates.

Why is it needed?

Makes needed UI and consistency improvements to the Template panel.

How to test

1. From your terminal app, run grunt build from within the _build/templates/default folder and clear your browser cache.
2. Create and update a few Templates to verify behavior and appearance is as expected.

Related issue(s)/PR(s)

Partial port of #15146