Refactor automatic emails

[2ynn]

Type of Issue: Code refactor & New Feature

Currently our automatic emails are hardcoded in different places (signals.py , admin.py) and it is not possible for core members to have control on their content.

Expected Behavior:

• utility functions should be clarified and gathered in a single place
• emails should be bilingual and formatted uniformly
• core members should be able to draft email content (french/english) from the admin panel
• core members should be able to enable or disable an email type

[2ynn]

Feature request from core members: After each fruit group: ability to send an email to all pickleaders, inviting them to adopt freshly created harvests.

I think #91 could be relevant here, or at least a way for pickleaders to "unsubscribe" ?