Set permissions with checkboxes, not yes/no inputs

The yes/no pattern didn’t work too well, because:
- it didn’t read naturally as a question and answer
- often users left them completely unclicked if they didn’t want to set
  the permission (rather than clicking no)

This commit changes both the invite and edit user pages to use
checkboxes to set permissions. If also rewords these pages to read more
naturally, and explain what the permissions mean.

This meant changing some of the view logic around invites and
persmissions, and I ended up refactoring a bunch of it because I found
it hard to understand what was going on.

app/main/views/styleguide.py

@@ -1,7 +1,6 @@
 from flask import render_template, current_app, abort
 from flask_wtf import Form
 from wtforms import StringField, PasswordField, TextAreaField, FileField, validators
-from app.main.forms import CustomRadioField
 from utils.template import Template
 from app.main import main
 
@@ -18,14 +17,11 @@ def styleguide():
         code = StringField('Enter code')
         message = TextAreaField(u'Message')
         file_upload = FileField('Upload a CSV file to add your recipients’ details')
-        manage_service = CustomRadioField('Manage service', choices=[('yes', 'yes'), ('no', 'no')])
-        manage_templates = CustomRadioField('Manage templates', choices=[('yes', 'yes'), ('no', 'no')])
 
     sms = "Your vehicle tax for ((registration number)) is due on ((date)). Renew online at www.gov.uk/vehicle-tax"
 
     form = FormExamples()
     form.message.data = sms
-    form.manage_service.data = 'yes'
     form.validate()
 
     template = Template({'content': sms})