feat: allow using details and summary HTML tags

[CefBoud]

Description

Allows the usage of details and summary HTML tags.

Supporting information

BB-8726

Testing instructions

Create an Xblock and add the following HTML code

<details class="custom-accordion"> <summary> Conflicts of interest</summary> 
Some details. And then some more.
</details>

The HTML tags are escaped.

When using this PR's branch instead, the tags are taken into account.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.65%. Comparing base (018c36e) to head (29dfd14).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   62.65%   62.65%           
=======================================
  Files           4        4           
  Lines         166      166           
  Branches       15       15           
=======================================
  Hits          104      104           
  Misses         61       61           
  Partials        1        1           

Flag	Coverage Δ
unittests	62.65% <100.00%> (ø)

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[Agrendalath]

👍

• I tested this: checked that the tags are rendered correctly
• I read through the code
• I checked for accessibility issues: n/a
• Includes documentation: n/a
• I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository