Implement username availability checker

Outlined here: https://github.com/vector-im/riot-web/issues/3605#issuecomment-298679388

```HTTP
GET /_matrix/.../register/available
{
    "username": "desiredlocalpart123"
}
```

If available, the response looks like
```HTTP
HTTP/1.1 200 OK
{
    "available": true
}
```

Otherwise,
```HTTP
HTTP/1.1 429
{
    "errcode": "M_LIMIT_EXCEEDED",
    "error": "Too Many Requests",
    "retry_after_ms": 2000
}
```
or
```HTTP
HTTP/1.1 400
{
    "errcode": "M_USER_IN_USE",
    "error": "User ID already taken."
}

```
or
```HTTP
HTTP/1.1 400
{
    "errcode": "M_INVALID_USERNAME",
    "error": "Some reason for username being invalid"
}
```