update twitter references to x

[BornDeleuze]

In response to this issue

All lowercase instances of twitter replaced with x, while uppercase instances replaced with X. The data from surveys in csv files were not changed.

This update will not work without the changes to the styleguide here

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[BornDeleuze]

So I figured I'd try to fully test it out, which I hadn't done as it's been a while since linking style repos. I went ahead and linked this project with ember-styleguides locally with pnpm link, and noticed we didn't have a twitter svg on this site. I added that in here and it looks pretty good.

To get this to play nicely locally I had to add ember-svg-jar as a dependency, but I believe that this isn't necessary for prod, so did not include that. I'm not fully sure what the build process is so I could be off!

The footer also has an x svg, but this is loaded from ember-styleguides. I could not get that logo to load properly, so added a review on that PR to use the svg i added here.