Revamp 3PID invite notifications mechanism

[maxidorius]

Currently and by default, we offer to contact the pre-configured central IS server when a client attempts to validate a remote 3PID, regardless if the session is for 3PID binding or just for validation (like for authentication).

We need to change this mechanism so:

• Successfully validating a session does not trigger anything else
• Congratulate on 3PID binding via 3PID notification
• Offer remote 3PID binding within the notification of 3PID binding, instead of outside of it so it flows naturally
• Central IS servers are no longer enabled by default

[maxidorius]

Remote sessions and bindings will be dropped altogether for the following reasons:

• They do not add a lot of value but are very confusing for users, given reports received over the years
• The encourage the current status quo of a central server
• They promote sending personal identifiable data to a remote server
• They do not fit with the core aspect that mxisd is to be used for self-hosted setups with their own dedicated domains.