Support for user-triggered events in the pRMT app

[yatharthranjan]

These can be added at the source level with some default functionality. Each source can also override the functionality if required. These can be shown as a separate button on the UI source table for each source. It should also be able to disable per source. We should also be able to provide some context for each event, but need to investigate what is the best way to achieve that.

See discussion in RADAR-base/RADAR-Schemas#386 for more info.