fix performance issue in HtmlToPlainText#convert_to_text

If an empty link <a href="whatever"></a> is followed by more text, the
regular expression would take ages to complete its matching.

We originally found this issue when we found a background job that had
been running this regular expression at 100% CPU for 6 days.

(i made a test addition for this, without the code change you can see how the test takes an extremely long time to complete. not sure how useful of a test that is.)