Make all extracted start and end datetime stamps time zone aware.

[DinoBektesevic]

Standardizers extract start and end datetime stamps from headers of uploaded FITS files. The best I can read out from the FITS standard used by Astropy there is no particular set reference time zone in which the dates are expressed in:

As specified by Bunclark & Rots (1997), time zones are explicitly not supported in FITS and, consequently, appending the letter ‘Z’ to a FITS ISO-8601 string is not allowed. The rationale for this rule is that its role in the ISO standard is that of a time zone indicator, not a time scale indicator. As the concept of a time zone is not supported in FITS, the use of time zone indicator is inappropriate

when they are expressed as a datetime stamp for which this information is ambiguous. Really I don't know why we would need to solve this problem instead of just inserting the raw data as-is into our DB but unfortunately, Django ORM datetime stamp is sensitive to differences between naive and non-naive timestamps.

Setting USE_TZ=False in settings.py uses naive timestamps, no timezone information, and vice-versa. FITS files we're testing with come with both naive and non-naive timestamps. It's a matter of choice whether we want to go through the extra effort to ensure all timestamps are timezone aware or not. This change affects Standardizer metadata processing.