* Extracting container for allowed tags & attrs

Prior to this commit, we had several different locations in which we
specified ALLOWED_TAGS and ALLOWED_ATTRIBUTES for HTML rendering and
sanitization.

Curious to see how these either intersected or didn't, I opted to
create a container module that allows for us to more readily normalize
these allowed tags and attributes.  It's possible that we won't do any
normalization, but this work helps make that easier.

Ideally, I'd love us to contextualize "why did we choose the
tags/attributes we chose?"  But for now, I think consolidating these
tags and attributes will help make adding a `details` and `summary` tag
easier.

This relates to forem/rfcs#296

See [Google Sheet][1] for analysis of what tags/attributes are used, the
intersection and union.

[1]:https://docs.google.com/spreadsheets/d/1yj-a1qus1o0o4cj-_gOMP5yteeg-_f3s5z7kvK0Y7RM/edit#gid=0

* Fixing misnamed constant

* Fixing misnamed constant

* Extracting additional HtmlRendering use cases

* Adding comparative documentation for HTML tags

* Fixing broken parameter signature

* Moving constants into MarkdownProcessor