This commit adds a Gradle task to generate .ms templates from .mjml templates. The main goal is to be able to send responsive HTML emails.

This commit fixes the exceptions caused by additions to the email data created with Map.of (which returns an immutable Map)

This commit adds the support for rendering multipart emails, rendering both plain text  and HTML versions of the email. It also has the first HTML template for the confirmation email for organizers.

This commit removes the overridable attribute from the TemplateResource constructor as it was set to true in all of them (except the deprecated ones). The overridable property is always set to true, but might be removed in the future.

This commit adds the configuration key to enable HTML emails at system, organization and event levels. The default value is true. When disabled, only plain text templates will be rendered, HTML templates will be omitted.

Make the look and feel similar to the public site design

Prior to this commit, only the plain text message was included in the checksum. This commit also takes the HTML message into account, to avoid false positives in the checksum comparison

Avoid email tracking and rewriting on email links. More here: https://app.mailjet.com/support/how-to-exclude-a-link-from-being-tracked-when-click-tracking-is-enabled,721.htm