[development] define critical bug

[qomo]

I suggest we define what a critical bug is, so that we don't have too
many reported.   Yesterday I saw a thread where a developer downgraded a
critical bug to normal, and the user said it seems critical to her.
(something like drupal wouldn't process dates for an event before
1970.)  

chx and dries' exhortations have really brought down the number of bugs,
which is great.  however, to get down to zero, we need to clear them out
faster than they are being reported.

Ann