Fix word counter to work with multiple instances

On loading the page the word counter only got initialised on the first instance of a `textarea`. This meant that if there was more than one `textarea` on the page it would only show the word count when that `textarea`'s value was changed.

This commit changes the behaviour so that the word counter is initialised for each `textarea` on page load.

Fixes [card #7905](https://www.agileplannerapp.com/boards/187120/cards/7905).