Refactor AJAX controller to use plugin manager

[demiankatz]

The AJAX controller is currently overcomplicated and has too many dependencies. This PR is intended to refactor it to use a plugin manager so that the separate AJAX routines can be isolated from one another, and new actions can be more easily added.

TODO

• Discuss whether there is a better way to handle the "no session write" functionality
• Finish refactoring existing methods
• Make sure all tests still pass
• Write documentation

[demiankatz]

@EreMaijala, I believe that this refactoring is now complete, and all tests are passing. Since you've had a fairly significant hand in some of the past AJAX work, I'm interested to hear your thoughts on this final version. If you have any suggestions for changing the interface of the AjaxHandler classes or if you dislike any of the changes to the AjaxController, please let me know.

I know there are a ton of files in this diff, but you can ignore most of the code in the AjaxHandler namespace -- once you've looked at one or two of those, you'll get the basic idea of how they work.

[EreMaijala]

Looks good to me. I tested by converting one of our Ajax methods to the plugin manager style and got it to work with relatively little fuzz.

[demiankatz]

Thank, @EreMaijala! I'll merge it today.