Fix email service in Shifts

[njlxyaoxinwei]

• Line-breaks are missing in the email template: we need to rename .erb to .text.erb
• Some Mailer methods are not working.

[njlxyaoxinwei]

A list of emails:

• [x] shift reports
• [x] sub_created_notify
• [x] sub_taken_watch
• [x] sub_taken_notification
• [x] payform_item_modify_notification
• [x] payform_item_deletion_notification
• [x] due_payform_remainder
• [x] late_payform_warning
• [x] email_stats
• [x] stale_shift

[njlxyaoxinwei]

Now all these emails work locally when called in the console!

[njlxyaoxinwei]

Fixed by PR #450