[ROOTS] Set up ROOTS Platform – placeholder 'example.com' used instead of actual domain in admin links and email invitations

[sblindow]

behaviour:
Some buttons contain links with a placeholder domain. For example instead of "roots-stage.liqd.net" they contain "example.com". During testing I have encountered this in two places:

1. Clicking "Auf der Website Anzeigen" in django-admin interface to view organisations/ projects etc. on the site
2. accepting an invitation to a private project or a moderation invitation (by clicking the link in the e-mail)

[goapunk]

This just needs to be changed in wagtail and django-admin, so can be done by anyone

[partizipation]

re. what Julian said, i think then someone would just fix this for prod only when setting up