NXCALS-2995 Variable statistics

@acc-logging-team - please have a look for approval.

Add variable statistics component, front-end service and feature integration. This allows a user to request the statistics of a variable to be displayed. This can be done from a variable search result or for the currently selected variables.

The statistics will be requested and displayed for all the defined time ranges.

The context sidebar layout was also reorganized to add support for action menus and cleaner "add" buttons.

Closes NXCALS-2995