Refresh: use Firefox brand fonts on some Firefox pages [fix #15542]

[craigcook]

One-line summary

Keeps Firefox branding on whatsnew, welcome, and campaign pages (mainly for the fonts).

This is admittedly a bit hacky and loads extra fonts so it's a performance hit, but doing it "properly" would probably entail a more significant refactoring of the nav and footer as well as some global styles. We should tackle that at another time.

Issue / Bugzilla link

#15542

Testing

Test a handful of whatsnew and welcome pages with the M24_WEBSITE_REFRESH switch on; make sure the text is rendered in Metropolis and Inter. The footer should use the new Mozilla fonts (only English gets the new footer; other languages get the old footer for now).

http://localhost:8000/firefox/125.0/whatsnew/
http://localhost:8000/firefox/132.0/whatsnew/
http://localhost:8000/firefox/133.0/whatsnew/
http://localhost:8000/firefox/welcome/1/
http://localhost:8000/firefox/welcome/10/

Firefox for Families page; make sure the page body has Metropolis and Inter, and the nav and footer render in Mozilla fonts (and don't otherwise break).

http://localhost:8000/firefox/family/

Nothing Personal page should use Firefox fonts and Mozilla fonts in the footer (this page has no nav):

http://localhost:8000/firefox/nothing-personal/

Challenge the Default page should use Firefox fonts, but this is an odd one because currently it's not available in English but the new Mozilla fonts are English-only. That logic will change soon so this is a pre-emptive strike.

http://localhost:8000/firefox/challenge-the-default/

[janbrasna]

There are also firstrun pages for some channels:

1. https://www.mozilla.org/en-US/firefox/123.0a2/firstrun/
2. https://www.mozilla.org/en-US/firefox/nightly/firstrun/

[stephaniehobson]

A deceptively easy solution!

I think we should merge #15571 and then rebase this to use the same switches that look for locale activation.

[craigcook]

There are also firstrun pages for some channels:

Oh good catch, I'll update these as well.

[craigcook]

Nightly WNP: http://localhost:8000/firefox/135.0a1/whatsnew/
Nightly firstrun: http://localhost:8000/firefox/nightly/firstrun/
Dev Edition WNP: http://localhost:8000/firefox/134.0a2/whatsnew/
Dev Edition firstrun: http://localhost:8000/firefox/134.0a2/firstrun/

[janbrasna]

To complete the list to test, there's also the non-MDN FXDE WNP (=for older versions): http://localhost:8000/firefox/99.0a2/whatsnew/

[stephaniehobson]

This is working great. The nothing personal page is missing the site_css block. I added it locally and it works fine to once it's added here it's good to merge.

R+ wc