upgrade to ES6

[NovemLinguae]

Any objections to upgrading from ES5 to ES6? This will break really old browsers such as...

• Chrome <= 20
• Safari <= 7
• Firefox <= 5
• Opera <= 12.1
• Internet Explorer <= 10

Complete list

... but this should affect almost no one, and will make coding easier.

[primefac]

At a certain point we must move on. Make it so.

[siddharthvp]

Actually, this will not break any browser. Since https://gerrit.wikimedia.org/r/c/mediawiki/core/+/902706/ earlier this year, no gadgets or core/extension-loaded javascript are served to ES5 browsers.