Fixup CTD landing & update promo link

[janbrasna]

One-line summary

Fixes link from /de home leading to CTD landing. (Also fixes random markup issues.)

Significant changes and points to review

The CTD was created in several phases and lived in a few places before settling where it is today, accumulating some fragments of its history that break things today. This aims to make it all work as expected:

Landing changed to fx base and cleaned up to just extend from base. Fixed:

• missing title/suffix whitespace,
• meta whitespace/newlines breaking some social previews,
• removed social image definition to enable the locale includes provide that.

Also fixed:

• Missing v6 heading in /fr shimmed with control (if you have the correct string feel free to suggest…)
• Some unclosed tags fixed by moving things around, some extra closing tags removed.
• Repetitive id on presentational elements fixed by adding size grouping in the id.
• Mobile markup rendering as text being sanitised marked as safe.
• Style imports were reordered for all the components to have the necessary variables defined, and anything unused removed.
• Added cursor: pointer to details summary.

Issue / Bugzilla link

#14957
#14950

Testing

http://localhost:8000/de/
http://localhost:8000/de/firefox/challenge-the-default/
http://localhost:8000/fr/firefox/challenge-the-default/?v=6
http://localhost:8000/pl/firefox/challenge-the-default/

[janbrasna]

This is a /de/–only promo include with hardcoded de strings, so I felt it's okay to also hardcode a /de/* link, given it's not available in the defined url()s otherwise.

[janbrasna]

BTW the only thing I have left as-is is the switch logic that still implies the original /firefox takeover:

bedrock/bedrock/firefox/templates/firefox/challenge-the-default/landing-switch.html

Line 20 in 36e8e9c

{% include 'firefox/index.html' %}

which results in missing strings if the campaign is turned off, and the landing not removed and/or redirected away (see e.g. https://www.allizom.org/de/firefox/challenge-the-default/ …)

I contemplated monkey-patching a meta-redirect instead of said include, but the page should be removed or redirected from when the time comes instead of flipping the switch anyways now when it's not /firefox takeover, so I kept it as-is — maybe the switch logic can be removed completely to be on the safe side?

[janbrasna]

Q: would it be worth to fix #13357 by replacing these with some more plain ASCII like • bullet?

[stephaniehobson]

Needs a rebase 🙏

[janbrasna]

(I cowardly did a merge instead to better show the diff, it should only be tabindex additions and data-cta-type removals from the recently landed PRs.)