Indentation in plain text adds unwanted spaces around links

Indentation gap had been added in front of every text in the indented block,
which can be wrong in cases where the text is "split" by an anchor, for example
when added by magic-links. There was used wrong indentation gap in the plain
text version too (3 in UI versus 4 spaces in plain text per level).

This had been reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1538775