Add feature flagged support for issuing for IDNs, fixes#597.
This patch expects that clients have performed valid IDN2008 encoding on any label that includes unicode characters. Invalid encodings (including non-compatible IDN2003 encoding) will be rejected. No script-mixing or script exclusion checks are performed as we assume that if a name is resolvable that it conforms to the registrar's policies on these matters and if it uses non-standard scripts in sub-domains etc that browsers should be the ones choosing how to display those names.
Required a full update of the golang.org/x/net tree to pull in golang.org/x/net/idna, all test suites pass.
This PR replaces the `x/net/publicsuffix` package with `weppos/publicsuffix-go`.
The conversations that leaded to this decision are #1479 and #1374. To summarize the discussion, the main issue with `x/net/publicsuffix` is that the package compiles the list into the Go source code and doesn't provide a way to easily pull updates (e.g. by re-parsing the original PSL) unless the entire package is recompiled.
The PSL update frequency is almost daily, which makes very hard to recompile the official Golang package to stay up-to-date with all the changes. Moreover, Golang maintainers expressed some concerns about rebuilding and committing changes with a frequency that would keep the package in sync with the original PSL. See https://github.com/letsencrypt/boulder/issues/1374#issuecomment-182429297
`weppos/publicsuffix-go` contains a compiled version of the list that is updated weekly (or more frequently). Moreover, the package can read and parse a PSL from a String or a File which will effectively decouple the Boulder source code with the list itself. The main benefit is that it will be possible to update the definition by simply downloading the latest list and restarting the application (assuming the list is persisted in memory).
The `regID` parameter in the PA's `WillingToIssue` function was originally used for whitelisting purposes, but is not used any longer. This PR removes it.
Roland Bracewell Shoemaker
Simone Carletti
Ben Irving
Jacob Hoffman-Andrews