feat: Implement a modern footer

[mgax]

Fix https://github.com/ietf-tools/www/issues/377

• Move social media links from the header to the footer, in line with the existing links (which are now aligned to the right).
• Add snippet type "Footer Column". The columns are displayed above social links and the old footer links.
• On mobile, the columns are collapsed, and the headings are tagged with accessibility markup, to make them useful for screen readers.
• A bit of code is shared with the test coverage branch (https://github.com/ietf-tools/www/pull/395) but it should be easy to merge.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.51852% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 83.87%. Comparing base (4ad3143) to head (4865217). Report is 8 commits behind head on main.

Files	Patch %	Lines
ietf/utils/blocks.py	94.44%	1 Missing :warning:
ietf/utils/models.py	91.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #400 +/- ## ========================================== + Coverage 82.72% 83.87% +1.15% ========================================== Files 124 129 +5 Lines 2599 2710 +111 ========================================== + Hits 2150 2273 +123 + Misses 449 437 -12 ```

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[mgax]

@SharmaineLim thanks! I'll mark the PR as "Ready for review" so that IETF have time to review it too.