Add automated check for "broken-reference"

[koppor]

When cleaing up image names, uploaded images are merged - and marked as "broken"

dc7d8308e775cca7bfe8b046229e869594cf8e81

There should be a check for figure><img src="broken-reference" alt=""> inside .md files to see if images are missing.

Refs https://github.com/JabRef/user-documentation/issues/527