Strip footnotes from excerpts

fixes #4572

- Remove both inline and bottom footnotes from excerpt output before stripping the remaining HTML
- No more red errors, black text or bold links in codemirror, as codemirror gets confused by footnote syntax. This is a step towards the new editor which has no syntax highlighting in the editor
Merge pull request #4580 from ErisDS/footnote-excerpt

Strip footnotes from excerpts