Add config options to set arbitrary value for placeholders in 3PID notifications

[maxidorius]

To avoid forcing people to download built-in templates for notification, edit them and change the config to host them, it would be best to give configuration options to set arbitrary value for certain placeholders in 3PID notifications.

Typical use: change the landing page of a 3PID invite, so people end up on the webclient, instead of the Matrix domain or the mxisd domain, as the client would be hosted at <client>.domain.tld while the Matrix domain would be domain.tld and the mxisd hostname would be matrix.domain.tld.