Migrate app to TypeScript (#1637)

* Initial setup and migrated src/common

* WIP

* WIP

* WIP

* Main module basically finished

* Renderer process migrated

* Added CI step and some fixes

* Fixed remainder of issues and added proper ESLint config

* Fixed a couple issues

* Progress!

* Some more fixes

* Fixed a test

* Fix build step

* PR feedback

webpack.config.main.js

@@ -16,7 +16,7 @@ const base = require('./webpack.config.base');
 
 module.exports = merge(base, {
     entry: {
-        index: './src/main/main.js',
+        index: './src/main/main.ts',
         mainWindow: './src/main/preload/mainWindow.js',
         preload: './src/main/preload/mattermost.js',
         modalPreload: './src/main/preload/modalPreload.js',
@@ -28,7 +28,7 @@ module.exports = merge(base, {
     },
     module: {
         rules: [{
-            test: /\.js?$/,
+            test: /\.(js|ts)?$/,
             use: {
                 loader: 'babel-loader',
                 options: {