Replace HTMLEntities gem

We use https://github.com/threedaymonk/htmlentities gem in some places. Let's replace it.

- This gem is slow as hell when used improperly (instantiates the same regexp over and over again, and that involves parsing its source)
- This gem isn't thread-safe
- This gem is no longer maintained (last release in 2015)
- This gem fails to recognize entities in some edge cases (bug reported 3ys ago)

Ridiculously, this gem has 80 millions downloads…

Start by locating every use of the threedaymonk/htmlentities gem and checking how it is declared as a dependency. Compare the replacement against the gem's performance, thread-safety, maintenance, and entity-recognition concerns; done means the old gem is no longer used and affected behavior remains covered by the project's existing checks.

Written by the indexing model from the issue text.