pattern attribute specifies a regular expression the form control's value should match. If our generated password doesn't match it we should try other generated passwords.
TBD: how do we honour the requirements? Easiest would be to have many different formats available and test the pattern on each of them at different lengths until one works. e.g.
- current character classes
- add some symbols (see bug 1559986)
- remove numbers from the current classes
- remove letters from the current classes