Add character counter to editing project description form

I recently worked with @amara on changing the description of gitlab ce project (https://gitlab.com/gitlab-org/gitlab-ce/issues/35144)

We had to trial and error the new description to determine if (and if there was, what the maximum character count was). Something that was simple, took longer because it wasn't obvious. I would like us to consider adding a character counter to that input textarea, so that it's easy and clear for a user to know that he/she can only add a description of up to 250 characters (including spaces)