Font family update - Githubissues

This is a first stab at updating our fonts to match what is (I'm told, anyway; I haven't seen any actual example artifacts) being used in the physical exhibition. Mainly this involved:

Replacing Source Sans Pro with Open Sans for body text (subtitles, button labels, and card text)
Adding Rajdhani as our display font, used for the title bar title and the card titles
Adjusting various font sizes and weights and line-heights to make sure things still fit (Open Sans is larger than Source Sans at the same font-size specifications) and look okay

There might be more to do with all of this, but I wanted to get a first draft in before tomorrow morning in case @camillevilla is able to go to Hohbach so we could see what the changes look like on the physical wallscreens. Plus, it would be good to get reactions from the team in case people don't like the way this is looking...

Also, I didn't remove the Source Sans Pro font files or CSS specifications, even though we're no longer using them with this PR. I'm not sure it matters too much, but in theory a future wallscreen (for Rumsey or whatever) might want to go back to using Source Sans Pro (since it's the default Stanford font) rather than Open Sans/Rajdhani, since they are specific to the SVA installation.

Some after examples:

sul-dlss / wallscreens

Font family update #295