Create background script

[Bartleby2718]

1. Additional bundling through webpack-cli #43

• Probably the most important part of this pull request.
• After the CRA build(react-scripts build) is over, webpack is used to bundle node_modules and background.ts. The downside is that it takes twice as long as the previous build.
• It uses a separate tsconfig file--tsconfig.other.json--to transpile TS files because it requires that noEmit be false. Still, I minimized the difference by extending the existing tsconfig.json. Simplify tsconfig.other.json through inheritance #68
• In the process, I added ts-loader because it somehow works better than the existing awesome-typescript-loader.

2. Create folder upon installation #43

• The code is basically the same as saveClicked or loadClicked because they all check the existence and the validity of webmarkFolderId.

3. Keyboard shortcuts #13

• Basically the opposite of Remove shortcut settings #53
• Please test this on a Mac.

4. Refactorings #63 #66 #67

• Thanks to utils.ts, it is now very clear what happens in the popup (index.tsx).

[jeongm-in]

1. Additional bundling

• Need clarification on "works better." Does it mean performance improvement? Build time improvement?

2. On install folder creation

• confirmed folder is correctly created on install.
• new folder is created in whatever occasion it is gone.

3. Shortcut

• confirmed shortcuts are functional.
• also tested on Ubuntu 18.04.2 LTS with latest Google Chrome stable 75.0.3770.100-1, although it doesn't mean much.
• will test on OSX environment later tonight.

[Bartleby2718]

1. My bad. For some reason, I thought awesome-typescript-loader was included in the package but didn't work. It turns out it just wasn't installed. It works fine with awesome-typescript-loader once I npm install --save awesome-typescript-loader. Still, let's go with ts-loader since it it shorter and more popular on GitHub than awesome-typescript-loader, although I don't know about the performance difference.

2, 3. Thank you for the testings.

[jeongm-in]

Shortcut works fine as expected in OSX. Approve merge into master