Dropdowns for context selection

[westluke]

For the dashboard page of CPO, turns the "open editor" button into a split button with dropdown that lets you choose a context to open the new file with.

Also changes the "choose context" modal in the editor to give users the option to choose from whitelisted contexts.

The whitelisted contexts, as well as the default context, get stored in environment variables in .env

This also bumps the version of the react dependency so we can use React.Fragment

The styling on the dropdowns and the modal is really really minimal, I was told to let someone else handle that

[jpolitz]

Merging so we can test it out on horizon; I haven't reviewed in a ton of detail yet, but seeing it live will be useful.

[jpolitz]

Here's what I'm seeing.

On /editor:

On /:

It seems like process.env is empty in these contexts. I did a fresh clone and copied .env.example to .env. We will need to add these new env options to Heroku to deploy, but that's not part of the Selenium build.

I wonder if there's something different about your build (webpack or similar) that's templating into different places differently?

[jpolitz]

Suggestion: Explicitly template into index and into editor in server.js rather than relying on the magic of process.env with webpack/react builds, e.g.

https://github.com/brownplt/code.pyret.org/blob/horizon/src/server.js#L108

(Will need to also add a variable declaration in the appropriate HTML template file)