feat(backup-reports): mjml template

[b-Nollet]

Description

Backup reports using MJML.

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft

[julien-f]

low priority

Import them "dynamically":

const templates = new Map()
for (const name of ['markdown', 'mjml']) {
  // todo
}