Mobile menu icon

[whelmich]

Improve the mobile menu icon:

• show a cross icon when the menu is open, so it is clear it closes the menu
• animate between the hamburger and the cross icon, inspired by this example
• increase click area to make the button more user friendly

[clefebvre]

I get no button bg here

[clefebvre]

Write to root@linuxmint.com to get a slack invite.

[whelmich]

I get no button bg here

Have tried recompiling the scss files? (I didn't include the compiled style.css file in my PR; I could add this if you want)

Also you might have to run the build script, because mint-section-header.html has been updated.