Feature/activity email

This feature is laying out some groundwork for the Activity summary email, this is where it is at:

I have a balsamic mock up: https://loomio.mybalsamiq.com/projects/emailsummary/Email%20Summary

I have converted that mock up into a html template.  You can view this by clicking the big blue button on the 'Home/Dashboard' page.  This will fire an email to mailcatcher.

Talking with @jdoud at the retreat we realised the next steps will be to create a mock up for the email settings and to user test and get some feedback for this with the html template(which could be properly emailed from staging).

After this it would be safe to nail the logic, which will be very similar to the inbox logic.

@rdbartlett @robguthrie @HSalmon @s01us @mhjb @jonlemmon