Remember selected calendar period within session

[Flix6x]

From #449:

The date selection resets to the most recent available data when you load an asset or sensor page. I was thinking of adding the date selection to the session, so that the same period will load when a user switches to a different page.

I like a selected range to persist across these data plot pages. We have start_time and end_time in the session variables (see ui/utils/view_utils.py::render_flexmeasures_template and set_time_range_for_session) - maybe that is where this should be going?

We don't want it to be necessary to reload the page in order to set a session variable, so I was thinking we could have relevant API endpoints (like /chart or /chart_data), or a new dedicated API endpoint (called when selecting a calendar period), to set the session variable, instead of doing that when the server is called to load the page.

I believe the /chart_data endpoint is a good place.