feat(plugins): added better error message when trying to load data source plugin module that is missing datasource constructor

feat(plugins): made data source custom edit view into a directive instead of html path config in plugin.json

feat(plugins): changed what datasources should return, they should now return the datasource constructor

Conflicts:
	public/app/partials/sidemenu.html

(cloudwatch) reuse credentials

Color the annotation bolt (#3590)

closes #3590 

Conflicts:
	pkg/services/sqlstore/migrations/migrations.go

Makes it easier to distinguish between annotations while
editing and using them; Uses the icon color.

Merge branch 'ssl-for-mysql' of https://github.com/improbable-io/grafana into improbable-io-ssl-for-mysql

Remove warnings when scanning for plugins in the default
catalog data/plugins

closes #3663