Add post tag UI autocomplete

[pkattera]

As a user,
I want to be able to easily input tags,

---

Let's modify our tags input editor to make tagging posts more intuitive.

Using a library like https://github.com/olahol/react-tagsinput will help us to set expectations around the tag without having to use error messages like this from #1678

See demo here: https://olahol.github.io/react-tagsinput/

It also sets us up to include auto-suggestions based on popular tags in the future.

[caleblogan]

@relativityboy Are you still working on this? Related #2437

[pkattera]

Acceptance criteria

• Create a new tag when the user hits the space bar, hits a comma, or presses enter

[bnchdrff]

port our existing validations to whichever tag editor we end up using; no additional validation

[roadscape]

Suggestion from #2679:

I would like to see my most used tags (posting and reading) show up first.
Ability to order/manipulate tags that show up in Tags column would be useful.