Make mail content templatable

[dArignac]

The mail content of subscriptions is currently created from 2 settings variables and some variable replacements.
This should really be a template to allow customization.

[tbaker4802]

This can be modified by modifying the PRICE_MONITOR_I18N_EMAIL_NOTIFICATION_BODY setting inside the app_settings.py

I ended up creating a view that corresponds to that setting that allows modification through a webform in the front end. takes about 10 minutes to do. I can push it if you want.