Add Enable/Disable checkbox to Triggers

Most other pieces of Bamboo, at this point, have granular on/off states; triggers remain one of the few places without.

The use case is a situation where there might be automated nightly builds under normal circumstances, but that at times need to be paused and only run manually.

• Disabling the plan prevents manual execution
• Deleting the trigger means recreation down the road
• Adding a manual build stage is hacky

To be fair, in our situation, I believe Bamboo 5 deployment plans may solve our immediate issue, but filing the improvement request anyway since its a nice to have option.