Plugin system

[simov]

Closes #1400
Closes #1406

I just merged #1400 and #1406 here + refactored all of the 4 plugins to use similar structure.

[nylen]

I like the concept, this is definitely the direction we want to head IMO. I'm a little worried about unexpectedly breaking things though, like #1409.

I have been swamped lately (just accepted a job with Automattic!) so I'll need at least a few days to do a proper review. @mikeal @FredKSchott @seanstrom etc. any help appreciated.

[simov]

I'm really not changing that much at this stage, just moving code around, the instance bug with auth is very silly, but apparently not caught by any of the tests.

[nylen]

I agree, I feel like I could have caught that in reviewing though, so I'd like to be more thorough this time.

[simov]

OK, my point in #1409 was that it's about code modified in this PR + I already merged two other PR's here, so I don't want to push even more things here :)

EDIT: Congrants on the new job @nylen !

[mikeal]

I don't have time to do a full review but I will say that I like this approach. I'll be impressed if you can actually work the forwarding logic in to a plugin, that logic always seemed to infect parts of the code I never thought I'd have to worry about :)

[simov]

@mikeal I'm not familiar with the entire code base in details yet tbh, but still the refactoring should start from somewhere.

[lalitkapoor]

@nylen off topic, but congrats on the new job!! :)

[seanstrom]

I'll be doing a thorough review today :)

[simov]

Thanks @seanstrom take a look at this PR #1406 too, as most of my latest ideas are there. Also not really that much changed at this point, I'm really just moving code into separate scopes as I see fit. For example the last one - the redirect one is just something that I thought it deserves it's own scope, and in fact except two of the fields in this newly created class are used externally, so it looks like a separate entity after all.