Components: Adding a generic token field component

[youknowriad]

In order to do #854 we need a generic Tags Input (with auto-complete...). Building such a component from scratch is not an easy task. So I shamelessly copied the component from Calypso and I adapted it to match our guidelines. It's a lot of code, so I'd appreciate help reviewing this component.

In order to test it. I'm using it in the sidebar with static values.
In a follow-up PR. I'll try to use it to update the post tags.

[nylen]

I do not think we should merge this without automated tests. Fortunately they already exist and we now have a PR to add component tests in this repository (#926).

Can we / should we remove some of the unneeded functionality here like a token's status?

[youknowriad]

Can we / should we remove some of the unneeded functionality here like a token's status ?

Sure we can, I don't know if we should? Couldn't this be useful somewhere else?

[youknowriad]

Tests are failing on CI but passing locally but the error is weird.

[youknowriad]

Tests are passing again. I'll appreciate some 👀 here. Thanks

[aduth]

When opening Post Settings on this branch, the page goes blank. I don't see the same on master.

[aduth]

Odd spacing. Previous variables make no effort in aligning by space.

[aduth]

Oh maybe that's related to #957. Could do for a rebase then.

[youknowriad]

@aduth Yep, it was related, should be fixed now.

[aduth]

I forgot how slow these tests were, but we're now seeing Travis timeouts in unrelated branches because they take too long. We can bump timeouts, but I'd almost rather remove the tests before allowing ourselves to succumb to accepting slow tests.

[nylen]

Even though these tests use enzyme, they look more like "integration tests" than "unit tests" and perhaps should not be run on every build. They might fit better in another category of tests (#939 (comment)). However, TokenField has a long history of weird bugs and these tests are pretty important.