feat: add configurable email templates

[gnyiri]

Description

Configure email templates from UI. The PR introduces a new table for storing email templates (name, description, subject, body). The logic of assigning email templates to status actions will be the same as before: the user selects the template name from the list of available templates (populated from DB).

Important notes:

• email handler classes will still be able to use the original approach (file based templates). If the email template of the given name don't exist in the DB the email service will try to look for a template file with the given name and use it as a template
• the migration will only be partial and a one-time manual step will be needed from STFC after deployment: the body/subject of the email templates have to be manually copied from the physical files so that the email templates will be stored in the DB (since I only have the template names and not the content).

Motivation and Context

At ELI we frequently change the email templates and found it uncomfortable to write physical files on the host of the backend.

How Has This Been Tested

Manual tests + backend test.

Fixes

N/A

Changes

See above

Depends on

N/A

Tests included/Docs Updated?

Yes

• [ x] I have added tests to cover my changes.
• All relevant doc has been updated

[jekabs-karklins]

❤️