Remove term ‘blacklist’ from codebase

‘Commonly used passwords’ is more specific, and avoids the terminology
‘blacklist’ which the National Cyber Security Centre explain to be
problematic:

> It's fairly common to say whitelisting and blacklisting to describe desirable and undesirable things in cyber security. For instance, when talking about which applications you will allow or deny on your corporate network; or deciding which bad passwords you want your users not to be able to use.
>
> However, there's an issue with the terminology. It only makes sense if you equate white with 'good, permitted, safe' and black with 'bad, dangerous, forbidden'. There are some obvious problems with this. So in the name of helping to stamp out racism in cyber security, we will
> avoid this casually pejorative wording on our website in the future. No, it's not the biggest issue in the world - but to borrow a slogan from elsewhere: every little helps.

– https://www.ncsc.gov.uk/blog-post/terminology-its-not-black-and-white

app/main/validators.py

@@ -9,18 +9,18 @@ from notifications_utils.recipients import (
 from notifications_utils.sanitise_text import SanitiseSMS
 from wtforms import ValidationError
 
-from app.main._blacklisted_passwords import blacklisted_passwords
+from app.main._commonly_used_passwords import commonly_used_passwords
 from app.utils import Spreadsheet, is_gov_user
 
 
-class Blacklist:
+class CommonlyUsedPassword:
     def __init__(self, message=None):
         if not message:
-            message = 'Password is blacklisted.'
+            message = 'Password is in list of commonly used passwords.'
         self.message = message
 
     def __call__(self, form, field):
-        if field.data in blacklisted_passwords:
+        if field.data in commonly_used_passwords:
             raise ValidationError(self.message)