* Federation: Implement Query Profile API

Implements the server portion of: `GET
/_matrix/federation/v1/query/profile?user_id=...&field=...`

Closes #278
Signed-off-by: NAndrew (anoa) <anoa@openmailbox.org>

* Properly export profile-related structs and fix wording
Signed-off-by: NAndrew (anoa) <anoa@openmailbox.org>

* Check provided user's domain matches our own.

* Implement shared secret registration

* Use HexString from gomatrixserverlib

* Correctly check username validility

* Add config for trusted ID servers

* Add new error

* Implement check for trusted ID server

* Complete unfinished comment

* Make comment more explicit in the config file

* Use go standard errors in membership.go

* Use standard errors instead of JSON responses in threepid

* Doc errors

* Remove unused parameter

* Stub APIs needed to let riot join a room

* Fix comments

Terse function names are used to make usage not stutter. For example:

```go
err := error.Forbidden("you shall not pass")
```

At the moment they are all the same fundamental `MatrixError` type. This will
be bad if we ever want to `switch` based on the kind of error. I'm hoping we
won't ever need to introspect into errors like this: ideally these errors would
be created purely for immediately being returned in an HTTP response.

`MatrixError` implements the `error` interface.