Add wagtail configuration for messaging

To test this, go into settings -> messaging settings.

Add some settings! Like ones for new submissions. The text box should auto populate with some stuff, and you can write your own message using the variables at the top. Do the same for subject and slack message (reused for zulip)

Then do things like create an application, and see your new email text get used.

Use the variables at the top to change the default system working. Please note that if you disable something like an email message, then it will STILL show up in the logging. You need to set the environment variable SEND_MESSSAGES to true so the email shows up in the logs, and set the zulip configuration so that things get sent to our zulip.