Special case /register for application services

[babolivier]

• Allow an application service to register usernames starting with underscores if its configuration file allows it.
• Support non-exclusive namespaces.

The latter means we shouldn't respond with Only 1 match allowed-like errors if a user ID provided during registration from an appservice matches more than one namespace and none of these namespaces is defined as exclusive. This requires checking whether a namespace is exclusive during registration.

[MohitKS5]

For the first part, should I assume that permission to use underscore will be implied by application service namespace 's regex or add a new parameter to config?

[rhymefororange]

Is this still an open issue? #1785 fixed the first part and #828 fixed the second, right? And #1599 mentions that #1784 is being used instead of this issue (which is closed).
Can someone clear up the confusion?

[rhymefororange]

@babolivier @kegsay any thoughts?

[kegsay]

Unsure, I haven't been keeping track of appservice support. If the tests pass, then it should be ok to close, though I don't know which sytests relate to this issue.