Keybindings for problem navigation

[spacerockzero]

Added

• "Next problem" keybinding: META + RETURN / CTRL + ENTER

• "Previous Problem" keybinding META + SHIFT + RETURN / CTRL + SHIFT + ENTER

• META should also be whatever the META key is on your OS. (CMD for OSX, Windows key for Windows)

• RETURN should be ENTER on Linux and Windows

Tested on:

OSX (10.12.4, 27" iMac)

• [x] Chrome 57
• [x] Firefox 53
• [x] Safari 10.1

iOS (iOS 10.3.1, iPhone 6 Plus )

• [x] mobile Safari 10.something
• [x] Mobile Chrome 58

Windows (Windows 10, Core 2 Quad Antique Gaming Rig)

• [x] Edge 14
• [x] IE11

Android (Nexus 10 Tablet, Android 5.1.1)

• [x] Chrome 57

Linux (Ubuntu 16, Core 2 Duo Antique Laptop)

• [x] Firefox 52
• [x] Chromium 57
• [x] Browser (User Agent says Chrome 53)

Temporary test-server

Can be tested at https://3ea02248.ngrok.io/ for a bit

[dllndv]

It would be better if the commands use ctrl on Linux and Windows. Using [windows key] + [Enter] feels weird.

[spacerockzero]

It's the idea to use CTRL on linux/windows, I just didn't test that yet. Will get my windows and linux machines on the case asap.