Twenty Seventeen: Unify labels for front page sections

[Clorith]

As reported by @ahortin in ​Alpha/Beta.

The Front page section # Content are labeled as such in the customizer Theme Options, but the preview calls them Panel # placeholder.

These should ideally have the same label to avoid any confusion, most likely the appropriate approach would be to re-label the preview ones to Front page section # placeholder, as this removed any confusion as to where the panels might come into play (and might be easier to translate I would imagine?).

[celloexpressions]

All of the code uses panel, but it looks like the customizer labels are all section. Section makes more sense to me, but if we have specific reasoning one way or another we can decide which way to go. Or, we could run both through some user tests. I'm also not convinced that either word is the right terminology to describe this.

[ahortin]

I think section works ok. My main concern was that I believe both the customizer lablels and the live priview labels should be the same, so as to avoid any confusion. So don't even use the word placeholder. If the customizer label is Front Page Section 1 Content then the live preview label should also be Front Page Section 1 Content so that users can clearly see that the customizer option and that particular section in the live preview are related.

[davidakennedy]

Changes panel placeholder language to be consistent with Customizer.

[davidakennedy]

I agree that using the same language here would help. 38567.1.patch​ does that.

[karmatosed]

In 39094:

Twenty Seventeen: Unifies the labels for front page sections

Front page sesctions # Content were labelled in Customizer Theme Options but the preview called it Panel # placeholder. This unifies that.

Props Clorith, ahortin, celloexpressions, davidakennedy
Fixes #38567