Customizer: Browse, Install, Preview Themes on Mobile

I tested the patch proposed on #37661 on Windows Phone 10 with the Edge browser. Here’s a visual record of that experience:

The biggest potential fixes I found were to show a loading indicator in the filters (next to the theme count button in the header?) when a new filter is loading, and to scroll the themes container to the top whenever a new section is opened or a search/filter is changed (which should be happening, but isn’t).

I should also note that since it is impossible to reorder images in galleries with Chrome on laptops that support touch, see #31652, I had to go to the text editor to fix one that jumped out of order when creating this post.

#customize, #customizer, #edge, #phone, #shiny-updates, #visual-record, #windows-phone