Add a twentyfifteen compat file for misc styles.

Props @iamtakashi

Also added an auto-rtl asset.
Add a twentyfifteen compat file for infiniscroll

This file is auto detected by
`the_neverending_home_page_theme_support()` which has an
`is_readable()` conditional.  We may want to switch that to a
predefined array to save checking the file on disk.  Unsure.

Props @iamtakashi

I also added a RTL version of the file.
Add RTL versions for infinite scroll compat files.