Unify all markup syntax to simple markdown syntax

Current markup syntaxes:

Wiki, annotations*, edit notes*, forums, Jira, etc. all possible syntax we have, those are internal and they even differ from each other, for example: sometimes the URL comes before, sometimes it comes after.

Markdown:

Markdown is an easy-to-read, easy-to-write plain text format, made for web writers: http://daringfireball.net/projects/markdown/

A widely accepted and more formal specification of Markdown is CommonMark: http://commonmark.org/

Many flavours of Markdown are listed at https://github.com/commonmark/CommonMark/wiki/Markdown-Flavors

Status:

Three reports have been made and attached to this issue by GCI students (see comments) to study existing Markdown variants, implementations, and their relevance to MetaBrainz needs.

Since the end of Feb 2018, a draft proposition (written in Markdown) for a flavor of Markdown dedicated to *Brainz projects, with specific features, equivalents, breaking changes, and migration paths is available at: https://gist.github.com/yvanzo/c3ffa4dd8410c2dbb4ff73bb41af8935
Reviews and comments are very welcome, see comments below.