Merge branch 'captcha' of github.com:matrix-org/synapse into develop

Captchas now work on registration. Missing x-forwarded-for config arg support. Missing reloading a new captcha on the web client / displaying a sensible error message.

Added a captcha config to the HS, to enable registration captcha checking and for the recaptcha private key.

Modified matrixService.register to specify if captcha results should be sent with the registration request. This is toggleable via useCaptcha in register-controller.

Validate power levels event changes. Change error messages to be more helpful. Fix bug where we checked the wrong power levels

BF: tab completion did not work with commands. $scope.input contained only the typed chars not the result of the completion.
Needed to fire an event so that ng update the input model

This reverts commit f286a4fc.

Show the message sending flow state in the messages list:
  - While sending, the message appears semi transparent in the chat.
  - If successfully sent, it appears as before, ie normal
  - In case of failure, it appears in red with an Unsent text.