C# 6 features/clean up

[ArcticEcho]

To summarise, I've:

• Used certain features (namely, string interpolation and expression-bodied properties/methods) of C# 6.0 where I feel it brings an improvement (to readability).
• Removed ChatbotAction's GetRegexMatchingPattern and GetRegexMatchingObject methods, as I feel they are redundant and can be optimised by using a single new property (RegexMatchingObject) which returns a compiled Regex object instead.
• Removed unused using directives.
• Reorganised all command/trigger classes' members in the following order (as there previously wasn't any order, that I'm aware of anyway): private fields, public properties (sub-ordered alphabetically) and then public methods.
• Changed all command/trigger methods to properties (besides RunAction) and removed the prefix "Get" (I'm still not sure why we were using methods to return simple strings in the first place).