Allow plugins to get their canonical web URL injected

This change allows a plugin to get its canonical web URL injected at
runtime.

The URL is composed of the server's canonical web URL and the plugin's
name, i.e. `http://review.example.com:8080/plugins/plugin-name/`.

The canonical web URL may be injected into any .jar plugin regardless
of whether or not the plugin provides an HTTP servlet.

Change-Id: Id1a8489d49bfebff4ffdb8de58e9d0445113a09b
4 files changed