Vim: Simplify create class snippet

We no longer add use strict at the top of our JS files because it is
added automatically by our ES6 transpiler. Additionally, I found it
difficult to remember to use this snippet, and think that it might be
easier to remember if it were scoped to the function that I would be
calling, which is React.createClass.
Vim: add React snippets to jsx.snippets

I pulled these from @wincent's snippets. I think they might be useful.

https://github.com/wincent/wincent/blob/e634059453246c0629d753192559cad3aeec14cb/.vim/ultisnips/javascript.snippets
Vim: Add snippet for JS function def with dynamic docblock

This is an experiment. I have been creating a lot of JavaScript
functions and then going back to add @param tags to document arguments
and their data types. I'm wondering if I can streamline this by using a
snippet.

This snippet will parse the arguments list and automatically create
@param tags for me in a JSDoc docblock. I made it smart enough to not
create a docblock if there are no arguments since it is easy enough to
generate one with docblock specific snippets if I want it for a
description or a @return tag or something.

I had wanted to make dynamic tabstops for the datatype portion of the
@param tags, but Ultisnips does not currently support this feature, so
I'll have to settle for manually going back and adding these.

Also, since this is an experiment for me, I decided to use the function
definition style that I use the most: functions that are properties of
objects.

- https://answers.launchpad.net/ultisnips/+question/187774
- https://answers.launchpad.net/ultisnips/+question/215812
- https://github.com/SirVer/ultisnips/issues/235