Twenty Fifteen: Improve naming and documentation of Customizer JS files

[celloexpressions]

js/customizer.js is really customize-preview.js - as it runs in the Customizer preview, not the main Customizer window. Also note that all core files use customize, not customizer in filenames, although themes generally don't follow that convention. Patch renames that file and updates the docs of that and js/color-scheme-control.js (which is really more of a customizer.js type of a file, but only needs to touch color schemes) to be more precise with respect to what those files are for. I imagine the docs will be improved further in #30149.

[celloexpressions]

Rename js/customizer.js to js/customize-preview.js and make some docs tweaks. I don't think the renaming part patched correctly.

[celloexpressions]

This one should work better.

[iandstewart]

With the above patch applied my local blog title and tagline are updating only in the customizer panel and not in the blog view. Pre-patch, everything is working correctly. @iamtakashi or @celloexpressions can you confirm?

[iamtakashi]

30160.2.diff works fine both in Customizer and the front to me.

[celloexpressions]

@iandstewart: may be a caching issue. Make sure customize-preview.js (from the theme) gets loaded properly in the Customizer preview iframe.

[iandstewart]

Looks like this patch is out of sync for me.

[iandstewart]

In 30271:

Twenty Fifteen: improve naming and documentation of customizer js files to match core filenaming conventions.

Props celloexpressions, fixes #30160.