RFR: Timer should not be sensor

## What? 

Timer is not a sensor anymore. It would now be part of st2 process (rules_engine for now but we can move if needed)

## Things fixed

* st2reactor/contrib doesn't exist anymore as webhook and timers are now moved.
* Registration of system trigger models now happen programatically (in webhook controller and st2timer) as opposed to bootstrap script based registration.
* Also with this PR, I refactored st2reactor/container/utils and moved trigger type creation method to st2common/services/triggers so it can be shared. 

## TODO
- [ ] https://github.com/StackStorm/st2/issues/913
Sweet, we can finally get rid of this "hack".