Markdown safely

[Jkrzy]

Description

First steps for #819, disabling attributes for rendered markdown and pre-rendering the HTML.

This doesn't address potentially layout breaking markdown, not quite sure how to tackle that at the moment.

[codecov-io]

Codecov Report

Merging #833 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #833      +/-   ##
==========================================
+ Coverage    90.9%   90.99%   +0.08%     
==========================================
  Files          39       39              
  Lines        1749     1754       +5     
==========================================
+ Hits         1590     1596       +6     
+ Misses        159      158       -1

Impacted Files	Coverage Δ
hours/utils.py	100% <0%> (ø)	⬆️
hours/views.py	87.68% <0%> (+0.06%)	⬆️
hours/models.py	96.7% <0%> (+0.15%)	⬆️

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