v1.18.0: ExternalMode, UID-only form

This release adds a few minor features:

• ExternalMode reboot, added by @anpep in #529.
• Support UID-only form in NormalizeUidGid for services and exec, added by @james-garner-canonical in #553. This was James' first contribution to Pebble -- thanks!

In addition, @IronCore864 refactored client code and tests to use the Requester interface in #549

This release also includes many documentation improvements from @IronCore864 and @dwilding. See our documentation site.

Full Changelog: v1.17.0...v1.18.0