Introduce GPG operations

Add methods for cryptographic authentication of Cask packages with
the GPG software suite.

Homebrew Cask invokes the system-standard `gpg` binary to import
keys to the GPG keychain, download signatures to versioned metadata
folders, and verify packages. Homebrew Cask aborts if any step fails.

GPG verification is not currently invoked during the audit or
installation processes, and is intentionally left undocumented.

Example usage:
    gpg = Cask::GpgCheck.new(cask)
    gpg.verify(package_archive_path) if gpg.available
re-use version in armitage
re-use version in astah-community
re-use version in colorport
re-use version in dynamiclyrics
re-use version in fing
re-use version in gog-downloader
re-use version in graphicconverter
re-use version in intensify-pro
re-use version in minimetro

and remove "static" from URL
re-use version in rythem
re-use version in treemaker
re-use version in uberpov
re-use version in videobox
re-use version in vuescan
re-use version in xslimmer
rename sqlite-database-browser.rb

to sqlitebrowser.rb, according to the naming conventions.
Refactor backend classes to match pre/postflight

From legacy forms "after_install", etc.
* abstract out `method_missing_message` to Cask::Utils
* provide `method_missing` coverage in pre/postflight blocks (fixes #7445)
* fix old-style rendering of Cask name as a classname in
  `method_missing` messages
Update dbeaver to 3.0.2
update license to oss
Upgrade SABnzbd+ to 0.7.20