Fix Markdown handling of # characters

• remove incorrect logic to identify headers
• ensure header detection abides by the CommonMark spec rules [1]
• remove isBugLink method, which is no longer needed since bug links
  (i.e. #number) are not recognized as a header by the new regex

Fixes 0022333

[1] http://spec.commonmark.org/0.27/#atx-headings

Signed-off-by: Damien Regad dregad@mantisbt.org

Changes to Joel's original code: squashed 2 commits, reworded commit
message as well as the comments within the code.