Issue 460 remember selected calendar period within session

[Flix6x]

I chose to set the session data with the /chart instead of the /chart_data endpoints (for sensors and assets), because the former endpoint can be called without the need to call the latter (using include_data=true). Right now both the sensor and asset page are calling the /chart endpoint with each date pick.

Closes #460.

[coveralls]

Pull Request Test Coverage Report for Build 2723799853

• 6 of 8 (75.0%) changed or added relevant lines in 3 files are covered.
• 94 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.04%) to 67.577%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
flexmeasures/api/dev/sensors.py	1	2	50.0%
flexmeasures/api/v3_0/assets.py	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
flexmeasures/api/dev/sensors.py	1	61.33%
flexmeasures/api/v3_0/assets.py	1	95.4%
flexmeasures/data/models/data_sources.py	3	52.56%
flexmeasures/utils/time_utils.py	16	76.55%
flexmeasures/api/v1_2/implementations.py	19	75.16%
flexmeasures/api/v3_0/sensors.py	19	70.26%
flexmeasures/api/v1_3/implementations.py	35	67.53%

Totals
Change from base Build 2711605930:	0.04%
Covered Lines:	7207
Relevant Lines:	10091

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💛 - Coveralls

[nhoening]

First question: why not reuse start_time and end_time?

(Can we retire them, as we used them on analytics and portfolio views?)

[Flix6x]

First question: why not reuse start_time and end_time?

(Can we retire them, as we used them on analytics and portfolio views?)

I didn't really want to involve / touch the code that re-interprets those datetimes in the server's ttimezone and stores them as datetime.datetime objects. I guess that was there for a different use case. I'm directly storing the ISO string in the session here.