Pin wtforms to 2.x

wtforms 3.0 is incompatible with our code. A separate issue will be
filed to address the incompatibilities, but this should be enough to get
working images out of it.

With 3.x, we're seeing:

```
  File "/home/horazont/Projects/python/snikket-web-portal/snikket_web/main.py", line 35, in LoginForm
    address = wtforms.TextField(
AttributeError: module 'wtforms' has no attribute 'TextField'
```

and the portal fails to start.

requirements.txt

@@ -5,4 +5,5 @@ hsluv~=0.0.2
 flask-babel~=1.0
 email-validator~=1.1
 environ-config~=20.0
+wtforms~=2.3
 typing-extensions

snikket_web/admin.py

@@ -63,6 +63,9 @@ async def users() -> str:
     )
 
 
+_LIMITED_ROLE_NAME = _("Limited")
+
+
 class EditUserForm(BaseForm):
     localpart = wtforms.StringField(
         _l("Login name"),
@@ -75,7 +78,9 @@ class EditUserForm(BaseForm):
     role = wtforms.RadioField(
         _l("Access Level"),
         choices=[
-            ("prosody:restricted", _("Limited")),
+            # NOTE: enable this only after something has been done which
+            # actually enforces the described restrictions :).
+            # ("prosody:restricted", _LIMITED_ROLE_NAME),
             ("prosody:normal", _l("Normal user")),
             ("prosody:admin", _l("Administrator")),
         ],