Totemize Indigo theme

First stab at making a clean new theme for Totem. This MR just makes the Indigo theme look a bit like Totem without doing too many changes. Next steps should include:

1. Deciding how important we find it to keep the look exactly as it was with the RG theme
2. Including translation for the few currently English strings in this theme (mostly in index_override.html)
3. Other fixes yet to be determined.

For context:

I have basically copied some stuff from the Totem theme made by RaccoonGang to the Indigo theme. The indigo theme, I think, is a better basis for us because it overrides almost nothing from the edX basis, whereas RaccoonGang maintains a copy of many edx-platform HTML files in their repo. Maintaining the theme they made would mean regularly checking if they made updates to it and merging that into our repository. I think maintenance efffort for a theme based on Indigo would be way lower. In that spirit, I would like to make edX "look like totem" without making too many changes to the edX original theme, so we don't have to port updates to the edx-platform HTML files to our theme repo. The advantage of using "Indigo" as a basis instead of the edx-bootstrap theme, is that it includes a script that integrates seamlessly with Tutor.