[FIX] web_editor: allow to copy paste when editing translations

When editing translations, you were not able to copy/paste texts before
this commit. Indeed, in some contexts, in some browsers, copy/pasting
text was creating a new paragraph inside the translation, preventing it
to be saved.

This commit simply adds code to remove these paragraphs each time text
is changed.

A more elegant solution will be found for master with website and
editor improvements.