RFE: Postorius should allow direct editing of the templates

I got the following report: « I get emails from mailman requesting approval for posts to the nodejs-sig list that tell me to "visit my dashboard" to approve or reject the email, but they don't include any link to the dashboard. »

It's true that currently Postorius only allows setting the URL for the message template. I think it should have an option to edit that template directly (with a small doc on the side listing placeholder variables). It could then store that value in the DB, serve it on a specific URL and set that URL in Mailman's REST API. Setting an URL manually should still be possible.

When a list is created, Postorious could register the list:admin:action:post.txt template with a modified message containing its own URL to avoid the problem reported initially. Ideally it would also register modified footer messages with links to HyperKitty if it is present.

This is related to #281 (closed) but broader.