edx.org/LMS: Use/Style Semantic Text in Login and Registration Banners

Per https://edx-wiki.atlassian.net/browse/LMS-1160, this work aims to revise the login and register (marketing edx.org redesign-driven) banner elements so that the text that is now contained in the rasterized image (using a bg replacement technique) is rendered and styled in similar fashion for the accessibility case of users being in High Contrast Mode.

This is also architected to provide theme-based support and avoid any graphical issues for page headers that do not require these graphical elements.