Refactor email content to partial

The only difference between the weekly and daily email was the use of 'day' or 'week', so might as well DRY it up a bit :+1:
Refactoring to make flow of text better

eg. instead of 'this day' => 'today'

instead of 'next day' => 'tomorrow'
DRY up the reminder mail spec

Use an array to half the amount of specs we need to write :+1:

Also add a more explicit check of the email contents to check our new time format changes
An excellent refactor provided by @tokland

Original SO post here: http://codereview.stackexchange.com/questions/72161/dry-ing-up-some-rspec/72168#72168
Fix _mail_content haml file with missing tags
Merge pull request #785 from petems/refactor_email_content_to_partials

DRY up the email partials