Light Mode (#139)

This is a PR that implements a site-wide light mode and dark mode. Here's a summary of the PR:
* implements  a `themify()` SASS mixin and a `$themes` object to generate themed SASS classes (based on parent class specificity) (@emmyc)
* adds themed variants to the editor/output (@emmyc) and to the profile panel + sketches pages (@malsf21)
* creates a `<Switch>` custom component that adds visual decoration to a simple textbox (@emmyc, @fan-matt)
* adds the `<Switch>` component to the profile page to toggle the user's preference (@emmyc)
* slightly modify's the Profile Panel's name input (@malsf21)
* creates Redux handlers for the user's theme state (@fan-matt)
* saves user's state to document-wide cookies (@malsf21)
* writes Jest/Enzyme tests for the `<Switch>` component (@malsf21)

Big thanks to @emmyc for taking the lead on this PR, and @fan-matt for helping me correct my small mistakes!

README.md

@@ -1,6 +1,6 @@
 ## ACM Teach LA's Editor Frontend
 
-[![Build Status](https://travis-ci.org/uclaacm/TeachLAFrontend.svg?branch=master)](https://travis-ci.org/uclaacm/TeachLAFrontend) 
+[![Build Status](https://travis-ci.org/uclaacm/TeachLAFrontend.svg?branch=master)](https://travis-ci.org/uclaacm/TeachLAFrontend)
 [![Netlify Status](https://api.netlify.com/api/v1/badges/15895bed-2a7e-4a27-aa63-633a0cd645f1/deploy-status)](https://app.netlify.com/sites/sleepy-franklin-7a3e4c/deploys)
 
 This repository holds the frontend code for the ACM Teach LA online editor! Teach LA uses the editor to help teach LA students about Python, Web Development, and expose them to computer science!
@@ -58,7 +58,7 @@ The client should now be automatically opened in your browser; however, you can
 
 ## Notes for Developers:
 
-* every time you pull from master, make sure to run `npm install` - it's likely that some dependency has changed!
-* `lint-staged` and `husky` auto-prettify some JS code on save - don't be spooked!
-* Travis CI auto-builds branches and PRs - make sure that `npm run test` `npm run prod_build` pass, or your changes for-sure won't work!
-* Netlify auto-deploys PRs, branches, and production deploys using the contents of `npm run prod_build`!
+- every time you pull from master, make sure to run `npm install` - it's likely that some dependency has changed!
+- `lint-staged` and `husky` auto-prettify some JS code on save - don't be spooked!
+- Travis CI auto-builds branches and PRs - make sure that `npm run test` `npm run prod_build` pass, or your changes for-sure won't work!
+- Netlify auto-deploys PRs, branches, and production deploys using the contents of `npm run prod_build`!