Fix timestamp handling in instxml and in data manipulation

[vladimir-mencl-eresearch]

Fix pending issues identified in #92 ( this comment ) to avoid eDB ignoring updates to Institution or Realm data because ts has not changed:

(1) For an Institution, where InstServer and ServiceLoc objects track their own timestamps, return institution ts as maximum over ts of institution itself and its subordinate objects.

(2) In data manipulation of subordinate objects included in the export, update Institution ts whenever a subordinate object is added, updated or deleted.

• This also covers removing an InstServer or ServiceLoc* which would not be covered by the max calculation.
• Achieve this via Django signals (these are also active for manipulations done via Django admin interface).
• Generally rely on post_save and post_delete signals.
• For Contact objects (linked via a ManyToManyField), use a pre_delete signal instead to still see the links.
• Guard Institution.institutiondetails navigation with InstitutionDetails.DoesNotExist (in case the details object does not exist).
• Suppress the signals from parse_institution_xml same as other update-related signals are suppressed (to avoid too high churn updating institution timestamp).

[vladimir-mencl-eresearch]

I have tested this carefully and it solves the issues with CAT eDB updates.

@zmousm , @ghalse , any comments on this? OK to merge?

[ghalse]

I've had a quick look through it, and it seems to make sense. But I'm not going to get a chance to properly test it for some time :(

[vladimir-mencl-eresearch]

Thanks @ghalse !

Has been now running in PROD at member.eduroam.net.nz for a week - with eDB ingesting the feed and timestamps being updated.