Complete templates for missing emails

[danieleguido]

Please correct the following email templates:
[ ] - The user requests a change of plan account_plan_change_to_educational.txt
[ ] - The user requests a change of plan account_plan_change_to_researcher.txt
[ ] - Accepted change of plan: account_plan_change_accepted.txt
[ ] - Rejected change of plan: account_plan_change_rejected.txt

[ ] - create an email when an user register and opt for the Student User Plan
[ ] - create an email when an user register and opt for the Academic User Plan

With best wishes,
The Impresso team