feature base class poc for standardized separation of concerns using Capabilities class

[diosmosis]

Description:

Adding this base class for three reasons:

• standardizing this practice will make it easier to break apart monolithic parts of the code (eg, making the troubleshooting functions use ajax)
• ajax functionality is not well handled currently (ajax handlers must always be added, even if a feature should not add other hooks)
• the code that initializes the different feature classes is currently rather unwieldy. the use of a standardized api will simplify it.

This refactor will be done piecemeal.

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed