Add Elasticsearch as datasource for Application logs

[ricoberger]

We are adding Elasticsearch as second datasource besides Prometheus.
Elasticsearch can be used to query the logs of our applications. For
that we extend the Applications CRD with a logs field, which can be used
to define Elasticsearch queries to retrieve the logs of an application.

In the kobs UI we added a new tab "logs" for the applications view,
where a user can select the defined queries for an application. When a
query is selected, the query is run against Elasticsearch and the logs
are shown in a table view. Next to the query we also run an aggragation,
so show the distribution of the logs in the selected time range in a bar
chart.

Next to the support of Elasticsearch in the applications view, we also
added an interface to query the configured Elasticsearch datasource
directly.

• I added a CHANGELOG entry for this change.