Set default socket timeout correctly (Fixes #12097)

Python socket timeouts are specified in seconds as floating point.  This
commit sets the default timeout to a more reasonable 3 seconds, allows
the specification of a floating point timeout value, removes the
assumption that the timeout is specified in milliseconds and documents
all of the aforementioned.
Support for configuring the upgrade check URL (See #12097)

This commit adds configurability to the upgrade check URL via the
OMERO.web configuration mechanism.  Upgrade checks in OMERO.web can now
be disabled by running:

    bin/omero config set omero.web.upgrades_url ''
Use existing "omero.upgrades.url" configuration option
Merge pull request #2176 from chris-allan/12097-socket-timeout

dev_5_0: Update upgrade check handling in OMERO.web