fix(admin): the Users pages keep an administrator in and say what their fields mean

- An administrator could block, delete or demote (unset Active or
  Superuser on) their own account, and with it the last one, leaving the
  panel without anyone to undo it. These are now refused.
- The user form showed "No password set." for users who have one: its
  password field never read the user's password. Its "Set password"
  link went to ../password/, a page this panel does not have. The form
  now says whether a password is set and links the page that sets it.
- "Staff status" said "can log into this admin site"; here it lets a
  user change and delete the template VMs given to them, and is now
  called Template editor.
- A quota of 0 counted as no limit, like -1, so a user's quota could not
  be taken away. Now -1 has no limit and 0 allows none; the help says
  so, and the labels give the units: memory in MiB, disk in GiB.
- A user's page for a user that does not exist answered with a server
  error instead of not found.
- The user list said "None" for a user who never logged in; it says
  "never", and dates fit their column.
- The password form's labels no longer break mid-word.

admin/forms.py

@@ -1,10 +1,7 @@
 from django import forms
 from django.contrib.auth import password_validation
-from django.contrib.auth.forms import ReadOnlyPasswordHashField
 from django.contrib.auth.models import Group, User
 from django.core.exceptions import ValidationError
-from django.urls import reverse_lazy
-from django.utils.text import format_lazy
 from django.utils.translation import gettext_lazy as _
 
 from accounts.models import UserAttributes
@@ -72,26 +69,8 @@ class UserForm(forms.ModelForm):
             "is_active",
             "is_superuser",
         ]
-
-    def __init__(self, *args, **kwargs):
-        super(UserForm, self).__init__(*args, **kwargs)
-        if self.instance.id:
-            password = ReadOnlyPasswordHashField(
-                label=_("Password"),
-                help_text=format_lazy(
-                    _(
-                        """Raw passwords are not stored, so there is no way to see this user's password, 
-                    but you can change the password using <a href='{}'>this form</a>."""
-                    ),
-                    reverse_lazy(
-                        "admin:user_update_password",
-                        args=[
-                            self.instance.id,
-                        ],
-                    ),
-                ),
-            )
-            self.fields["Password"] = password
+        labels = {"is_staff": _("Template editor")}
+        help_texts = {"is_staff": _("May change and delete the template VMs given to them")}
 
 
 class UserCreateForm(UserForm):
@@ -125,8 +104,7 @@ class UserCreateForm(UserForm):
             self.save_m2m()
         return user
 
-    class Meta:
-        model = User
+    class Meta(UserForm.Meta):
         fields = [
             "username",
             "password",