Fetch and render plugin license from Gerrit-CI

Enrich the plugin information with a new 'license' field that captures
the first 2 lines of the LICENSE file published by Gerrit-CI.

All Gerrit core plugins are associated with the Apache 2.0 license.

Whenever a plugin exposes a LICENSE file in its source code, the plugin
manager displays a "legal" icon that shows the license type on mouse
hover.

Also add test coverage for JenkinsCiPluginsRepository for fetching the
plugin information from a successful Jenkins Job build run.

Old plugin card : https://www.pasteboard.co/s87jpkcCcKxa.png
New plugin card : https://www.pasteboard.co/ZG3qMXgQqY81.png
Hover on license: https://www.pasteboard.co/cYAPqhlB2HZY.png

Change-Id: Ieb274a7f236c0c6d3a03db07838b5ea2a1926767
(cherry picked from commit 37f52d04052f6e763e45e74bd4c21f2947b8f981)