Upgrade to Go 1.16

[Seth Vargo]

TL;DR

Upgrade to Go 1.16

Design

In addition to standard testing, there are two features we should make use of:

• embed directive to embed the HTML and locale assets inside binaries (dramatically simplifies our dockerfiles)
• replace signalcontext with signal.NotfiyContext

[Seth Vargo]

ioutil is also deprecated: https://golang.org/pkg/io/ioutil