Twenty Seventeen: missing selective refresh for site title and tagline

[celloexpressions]

We should also add support for header images, but that'll need to wait for #38172. Patch likely conflicts with #38426, which should be committed first. Shouldn't be difficult to merge.

This is required to expose the visual edit icons exposed in #27403, see the corresponding forthcoming make/core post for details. Twenty Eleven through Sixteen added selective refresh for site title and tagline when it was reduced in WordPress 4.5. Patch is mostly copied from Twenty Fifteen.

@davidakennedy this should be in beta 1 if possible to coordinate with the forthcoming commit on #27403 this morning.

[celloexpressions]

Add selective refresh to the site title and tagline, based on Twenty Fifteen.

[davidakennedy]

In 38966:

Twenty Seventeen: Implement selective refresh for site title and tagline

Adding the opacity property to the site title prevents opacity from changing during selective refreshes in the Customizer preview.

Props celloexpressions.

Fixes #38513.