feat(plugins): better logging and handling of loading plugins, try to create plugins dir if it does not exist, fixes #3974

closes #3980

Fixes when panel repeat is removed by clearing the templating variables

fix template url for prometheus annotation editor

add support for screenshots in plugins.

Allow plugin creators to include screenshots of their plugin
in action. Primarily for use in Grafana.net info pages.

    The original panel should clear all the templating variables and
    look like it did before repeat was selected. Fixes #3831. Also
cleans up after a row repeat is removed

Conflicts:
	examples/nginx-app/module.js
	examples/nginx-app/partials/stream.html
	examples/nginx-app/plugin.json

Escape html in table panel

closes #3673

(prometheus) templating by query result