News abstraction

[ddabble]

Depends on #353, and should change base to dev after that PR has been merged.

Main features

• Made NewsBase abstract (e8f1a666585a8aeb70b1ecd3a84811a53708fba7)
  • ❗ Important note: ❗ The migration in this commit is fairly experimental; it has been tested multiple times locally (migrating both forward and backward), but not thoroughly. I personally think that it's most likely safe for deployment, though I'm not 100% sure, naturally.
• Refactored article and event list + detail pages (716f5cb8b75124c047be13e48e5955f7fa0e9a85)
• Refactored article and event edit pages (2fc33475f158195e22f139affbf840e56b3ca7d0)

Additional highlights

• Improved tab-focusing links in header (a94966958c50087f042c8836d947f1ac3a3cc379)
• Added "Skip to main content" button for tab navigation (9b6747216c56c11ab4a2fc12f7bff2cf05fb902f)
• Added priority field to Secret (c51d8d48a247293f785a124c141ca967495eda81)
• Added last_modified field to various models (a8080a1e66946412b69ea4c5ff7ba022a8f19c7f)
• Improved multiple Django admin classes (79ec603c0ea2c64aa9f38c22ede3e1c32ba57352, 30ecdfef5a23a7b348b5fdf1abb8c40195e38ad0 and a8118b1f46111983bf5d5c9ffacabf41de534f9e)

[codecov[bot]]

Codecov Report

Merging #354 (662c4e5) into dev (ae2d774) will decrease coverage by 0.90%. The diff coverage is 76.48%.

@@            Coverage Diff             @@
##              dev     #354      +/-   ##
==========================================
- Coverage   83.71%   82.80%   -0.91%     
==========================================
  Files         118      117       -1     
  Lines        3837     4140     +303     
==========================================
+ Hits         3212     3428     +216     
- Misses        625      712      +87     

Impacted Files	Coverage Δ
util/admin_utils.py	53.70% <50.98%> (-46.30%)	:arrow_down:
web/multilingual/formfields.py	90.90% <57.14%> (-2.64%)	:arrow_down:
util/html_utils.py	57.14% <60.86%> (+7.14%)	:arrow_up:
news/admin.py	65.41% <64.61%> (-34.59%)	:arrow_down:
faq/admin.py	81.48% <73.68%> (-18.52%)	:arrow_down:
internal/admin.py	92.59% <89.47%> (-7.41%)	:arrow_down:
news/models.py	94.70% <97.22%> (+0.21%)	:arrow_up:
contentbox/admin.py	100.00% <100.00%> (ø)
contentbox/models.py	100.00% <100.00%> (ø)
faq/models.py	100.00% <100.00%> (ø)
... and 15 more

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