Disable django's debug mode for the collect command

Django will cache all SQL queries in memory when running in debug mode.
This uses up a lot of memory, so work-around that by disabling debug mode
for the collect command.
Call django.db.reset_queries after each legislator has been processed

Django will normally call that function when a request finishes processing.
For the collect command that won't be done, of course, since there are no
requests, so do it manually.
Ensure date ranges are bound by legislatures so they make sense
Make sure the nature graph is bound by legislature dates too
Avoid adding the newly added date objects to date_ranges for json queries
detail-legislator: top-suppliers: prefix the supplier link with institution siglum
BaseCollector: Fixing retrieve_uri method

Trying to retrieve URL only if HTTP error is >= 499 and added
max retries to 10.