make the modal mostly its own thing

[Pomax]

Closes #2252, closes #2250, when copy changes have been made.

Note: the donate modal text is not tailored to the foundation site itself at all yet, so in terms of testing. Uncomment https://github.com/mozilla/foundation.mozilla.org/pull/2254/files#diff-bcb353d4a212e3c287b0d8d8c377f1c5R325 and then:

• does the code look sane?
• does the modal pop up on localhost:8000 after 10 seconds in the same way that it does for the PNI subsite?
• does the CSS seem to be doing the right thing for the main site?

[Pomax]

@mavis I folded the "moving the react-ga-proxy" into this PR mostly because it affects the modal directly.

addressed

[Pomax]

I've also updated the main page modal to use the "for now" phrasing mentioned in #2250 (comment) /cc @kristinashu

[kristinashu]

Yeah let's run will this that copy for now!

Looks like the pink button is less round in this version than on Privacy Not Included? Can you make the ends fully rounded?

[mmmavis]

Neattt! ✨ Please merge this PR after fixing donate button's border radius!

[mmmavis]

@kristinashu I fixed the button roundness plus adjusted button text from 14px to 16px as per mockup.

[kristinashu]

Looks good, thanks!