Upgrade Go to 1.7.3

Blog post: https://blog.golang.org/go1.7 (2016-08-15).

We are on 1.6.3 now.

Past tickets: #18333 (moved) 1.6.2, #19703 (moved) 1.6.3

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information