Refactor Campaigns and Templates to allow transactional emails, automations, and A/B tests

[wmnnd]

Current situation

At the moment, all email content is stored directly attached to a specific campaign. Templates only define certain aspects of that email such as the footer or theme colors. This makes it difficult to add A/B tests or one-off emails (i.e. transactional emails or automations)

Solution

The concept of a Template needs to be redefined. In the future, a Template is where email content (text/mjml/html) is stored. In addition to that, Layouts are introduced. Layouts are very similar to Templates but they have a content slot for the Template editor. Layouts can be plain text, html, or mjml. A Template may or may not use a Layout. Templates also include the email subject. HTML layouts are only compatible with HTML templates. MJML templates can be used with the MJML/Block editor as well as the Markdown editor.

Plain Text Example

Plain-text layouts must include the {{ content }} placeholder.

{{ content }}

---
The footer is part of the Layout

MJML Example

MJML Layouts must include the keila-content tag as a child of mj-body.

<mjml>
  <mj-head>
    <mj-attributes>
      <mj-all font-family="Comic Sans MS" />
    </mj-attributes>
  </mj-head>
  <mj-body>
    <keila-content />
    <mj-section>
      <mj-column>
        <mj-text>
          This is a Layout footer in MJML
        </mj-text>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>

Necessary Steps

• Existing Templates must be automatically migrated to MJML Layouts or migrated to "LegacyTemplates" for later deprecation.
• Cerberus layouts are dropped in favor of full MJML support or marked for later deprecation.
• Block editor needs to be either dropped, replaced with a new MJML editor, or migrated to MJML
• New MJML editor needs to have read-only support for the layout part of the email
• Markdown editor needs to be updated to work with MJML
• Plain text editor needs to be updated to work with Layouts
• Campaigns and Templates have a many-to-many relationship with potential annotations such as the rate at which they should be used (e.g. template 1 is sent to 30% of recipients, template 2 to 70%)