Refactor accounts (including API/Views) and yearly_revalidation_users command scripts - Centralize and tests accounts emails

• cleaned accounts API
• cleaned accounts views
• refactored yearly revalidation scripts
• removed code duplicates and unified code
• centralized account actions (i.e.: supervisee rejection, etc.)
• made sure tests still work
• centralized emails with an email messaging utility
• added complete test cases for all emails
• removed unused emails
• patched code exceptions
• simplified inheritance in accounts API
• improved error management in API with DRF

This MR does a cleanup of the command targeting yearly re-validation of users.

Fixes #577