Consider dropping support for Safari 10

[tnajdek]

• Libraries increasingly start dropping support which forces us to re-transpile their code
• Some features have to be disabled (e.g. locale-aware date parsing) due to lack of support
• According to can I use Safari 10 has 0.01% (+0.07% mobile)
• On desktop, users should be able to upgrade to High Sierra which comes with Safari 11

[dstillman]

We can definitely do this.

All Macs running 10.10 can be upgraded to 10.11, which supports Safari 11, and you'd have to have an iPhone 5 or an iPad (4th Generation), both released in September 2012, to not be able to upgrade to iOS 11. All other iOS devices compatible with Safari 10 can be upgraded.