Reword migration template

[babolivier]

This PR updates the migration template by:

• adding an Element-branded one
• converting them to Jinja2
• adding an HTML version to both templates
• changing the wording from

In the past, identity services did not take capitalization into account when creating and storing Matrix IDs. We’ve now updated this behavior so anyone can find you, no matter how your email is capitalized. As part of this recent update, the duplicate Matrix ID %(mxid)s is no longer associated with this e-mail address.

to

In the past, identity services took capitalisation into account when storing email addresses. This means Alice@example.com and alice@example.com would be considered to be two different addresses, and could be associated with different Matrix accounts. We’ve now updated this behaviour so anyone can find you, no matter how your email is capitalised. As part of this recent update, we've dissociated the Matrix account {{ mxid|safe }} from this e-mail address.

[erikjohnston]

Assuming we've checked that the emails render correctly

[babolivier]

Assuming we've checked that the emails render correctly

I have, by running the script on a test db and having it send emails to mailtrap.