send a we miss you message to inactive users

[jnsereko]

Description

The mejor additions are

1. Added a field to the User model to track the last reminder email sent date. Let's called last_reminder_sent.
2. Created a new endpoint to send reminder emails to inactive users. This endpoint will take the list of inactive users generated by the existing /users/inactive-users endpoint and send reminder emails to them
   cc @MutegekiHenry

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Trello Ticket ID

https://trello.com/c/eIQVCw2r/1546-work-on-automated-emails

How Can This Be Tested?

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules