Add "normal" priority for issues (BB-3890)

Bitbucket's issue tracker defaults to "major" and there is nothing between "major" and "minor", both of which have connotation of either elevated or reduced importance.

I have gone to great lengths to teach PMs that not every issue is major/critical. Most issues deserve a priority something like "normal", so that elevated priorities like "major" or "critical" actually have some meaning.

I propose that a new status/priority of "normal" be added in the middle and that this become the default for all issues.

Thanks!
-JS