[SDP-1320] Refactor message.Message and its usage

marcelosalloum commented 1 month ago

Refactor message. The changes include:

DRY: Share the email style throughout the email templates
Rename template-related objects, methods, and files to make it explicit they refer to Staff and/or Email
Rename message.Message.Message to message.Message.Body for improved clarity
Update email templates by setting it's colors to black & white

[x] This PR has a reasonably narrow scope (if not, break it down into smaller PRs).
[x] This PR title and description are clear enough for anyone to review it.
[x] This PR does not mix refactoring changes with feature changes (split into two PRs otherwise).

[x] No new CONFIG variables are required -OR- the new required ones were added to the helmchart's values.yaml file.
[x] No new CONFIG variables are required -OR- the new required ones were added to the deployments (pr-preview, dev, demo, prd).
[x] No new SECRETS variables are required -OR- the new required ones were mentioned in the helmchart's values.yaml file.
[x] No new SECRETS variables are required -OR- the new required ones were added to the deployments (pr-preview secrets, dev secrets, demo secrets, prd secrets).

[x] This is not a breaking change.
[x] This is ready for production.. If your PR is not ready for production, please consider opening additional complementary PRs using this one as the base. Only merge this into develop or main after it's ready for production!