Prompt to accept Identity Server policies on registration

[lampholder]

When do we want to do this?

Is it for anyone publishing a threepid to the IS? Or also for anyone who might want to use the IS in future to look up other users by their threepid?

[lampholder]

This will require a change to sydent and presumably the spec to expose policy documents

[turt2live]

we should try and reuse matrix-org/matrix-spec-proposals#1692 if possible

[nadonomy]

After validating today, latest comps are in Zeplin: https://zpl.io/brMdWo3

[jryans]

We have realised that this is a subset of what will be achieved by #10167, so we don't need both issues.