Give user an option to drop pending events when enabling an agent.

This is shown in a confirmation dialog introduced for enable/disable actions.

![dialog](https://cloud.githubusercontent.com/assets/10236/4314700/e63d0e10-3ee4-11e4-91be-d8fc45063c71.png)

Currently disabling an agent only means a pause of event flow, so when you think you want to turn off notification for the rest of a day while keeping data collection in upstream agents, you'll have to unlink the notification agent from the agent flow instead of disabling it.  Otherwise, as soon as you re-enable the notification agent you get overflown by a flood of pending events from the upstream in the queue when they are outdated and useless, just wasting API calls that is often capped by a limit.

So, here's my attempt to make the "disable" action really useful.
I made drop_pending_events an assignable attribute.