New features: custom Function Actions and Input Actions

I’ve been slowly refactoring parts of Mycodo to be able to import custom single-file modules. Currently custom Inputs, Outputs, and Functions are supported. I’ve been thinking about refactoring Function Actions to act similarly, so users can create their own actions. I’ve also been wanting to add the ability to add actions to Inputs, to be able to do things such as MQTT publishes every measurement, and such. I know there’s been a desire for these features in the community for a while, so I wanted to start a discussion in case anyone else had any input.