Refactor hex colour preview Javascript

This commit improves the code that previews a hex colour when setting up
or changing an email branding.

Specifically it:
- refactors the Javascript to conform to our patterns (module pattern,
  preprocessed with Gulp)
- makes the code work when there are multiple colour previews on one
  page

It also does some visual prettifying, because I couldn’t help myself…

app/main/forms.py

@@ -734,21 +734,18 @@ class ServiceUpdateEmailBranding(StripWhitespaceForm):
     domain = StringField('Domain')
     colour = StringField(
         'Colour',
-        render_kw={'onchange': 'update_colour(this)'},
         validators=[
             Regexp(regex="^$|^#(?:[0-9a-fA-F]{3}){1,2}$", message='Must be a valid color hex code')
         ]
     )
     banner_colour = StringField(
         'Banner colour',
-        render_kw={'onchange': 'update_colour(this)'},
         validators=[
             Regexp(regex="^$|^#(?:[0-9a-fA-F]{3}){1,2}$", message='Must be a valid color hex code')
         ]
     )
     single_id_colour = StringField(
         'Single identity colour',
-        render_kw={'onchange': 'update_colour(this)'},
         validators=[
             Regexp(regex="^$|^#(?:[0-9a-fA-F]{3}){1,2}$", message='Must be a valid color hex code')
         ]