Add existence check on pusher channels for logs

We plan to stop sending log updates to log channels which are not
occupied, because logs produce a quite high volume of messages. This
commit is first step in that direction, it adds:

* a simple web endpoint which can handle pusher webhooks and register
  a channel as either occupied or vacant
* metrics measuring how many log parts would we need to send and how
  many we can simply ignore
Add logging
Add permissions to execute script/server
No need to run many workers, it's jruby
Require logs in logs/app
Make pusher_channel public
Allow to disable log channels check
rack.errors should be a logger instance
Fix process log part spec
Actually implement ignoring parts, but allow to turn it on/off in config
Implement /uptime endpoint
Refactor channel_name to a method
Small refactoring of enabling/disabling channel occupation check
We don't need autoload
No need to set logger manually
Remove rerun
Actually use rack/ssl
Add sentry
Make redis key less obscure
GET /uptime, not POST /uptime