~ Feature/activity email

Balsamic mock up: https://loomio.mybalsamiq.com/projects/emailsummary/Email%20Summary
And settings page too: https://loomio.mybalsamiq.com/projects/emailsummary/Email%20Summary%20Settings
@robguthrie lost your latest changes with the mock up

### How to test remotely: ###

#### VISIT /beta-features and check 'activity_summary_email' ####
1. turn on Heroku scheduler to run the rake task `emails:send_activity_summary` every hour
2. set the `.days_to_send = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday",  "Friday", "Saturday"]`
3. set the `.hour_to_send = 1`
4. come back the next day and check it (or the following day)

### Where this is at: ###
- Settings page now complete with all tests and cucs passing
- Html email template complete
- New EmailPreference model and EmailPreferencesService object created with specs
- ActivitySumary object created
- Migration created to move users exiting preferences into new EmailPreference class
- Created rake task to be called each hour (this will just email subscribers with :next_activity_summary_sent_at matches the current hour)
- Cucs for template all passing
- Logic added to register changes in title or description as new activity
- Fixed inbox bug around positions not being shown as new motion activity
- Added text email template

### To do ###
- @mhjb, @jonlemmon to style

### To remove after alpha testing ###
- daily_activity_email.feature
- daily_activity_email_steps.rb
- daily_activity.html.haml
- daily_activity.text.haml
- self.to_subscribers! from send_activity_summary.rb
- daily_activity(user, activity, since_time) from user_mailer.rb
- subscribed_to_daily_activity_summary_email? from email_prefereences.rb
- scope :daily_activity_email_recipients from user.rb
- task :send_daily_summary from emails.rake
- all refs in en.yml