Improve PO rules: Add check for square bracket count

Our beautiful translators are human after all, and sometimes they don’t copy the ikiwiki syntax quite right.

A simple check could be added to make sure the source and target strings have the same amount of [ and ] characters.

Reasoning:

- If there is an error on the number of brackets, ikiwiki will build without errors but the interface will be broken.

Original created by @emmapeel on 15454 (Redmine)