Server-side code for accepting crash reports and feedback

* Moved some shared code for copying InputStreams to OutputStreams into a utility class
* Modified the  dev report sender to send one report per connection
  * Easier to handle on the server side
  * If the connection fails after sending any reports, they don't need to be resent
  * Tor will reuse the circuit, so it's cheap
* Added server-side code for accepting dev reports
  * We need to protect the server's resources from DoS attacks
  * Reports can't be larger than 1 MB
  * Connections are limited to an average rate of one per minute
  * The rate limiter uses a token bucket to allow bursts of up to 1,000 connections
  * If the rate limit is exceeded, connection attempts will fail - clients will retry next time they sign in
  * The limits can be raised when we move to a bigger server (and when we have some users)


See merge request !288