Add form validation for max service and org name

There was a recent error in the logs because a service tried to change
its name to one exceeding 255 characters (which is a limit on the
database field). We can easily catch these errors on the form, so that
the user doesn't see an error page.

app/main/forms.py

@@ -1106,7 +1106,8 @@ class RenameServiceForm(StripWhitespaceForm):
         u'Service name',
         validators=[
             DataRequired(message='Cannot be empty'),
-            MustContainAlphanumericCharacters()
+            MustContainAlphanumericCharacters(),
+            Length(max=255, message='Service name must be 255 characters or fewer')
         ])
 
 
@@ -1115,7 +1116,8 @@ class RenameOrganisationForm(StripWhitespaceForm):
         u'Organisation name',
         validators=[
             DataRequired(message='Cannot be empty'),
-            MustContainAlphanumericCharacters()
+            MustContainAlphanumericCharacters(),
+            Length(max=255, message='Organisation name must be 255 characters or fewer')
         ])
 
 
@@ -1223,7 +1225,8 @@ class CreateServiceForm(StripWhitespaceForm):
         "What’s your service called?",
         validators=[
             DataRequired(message='Cannot be empty'),
-            MustContainAlphanumericCharacters()
+            MustContainAlphanumericCharacters(),
+            Length(max=255, message='Service name must be 255 characters or fewer')
         ])
     organisation_type = OrganisationTypeField('Who runs this service?')