Update release notes for v1.7.3

And freshen up the upgrading guide
Fix parsing issue in strtotime()

PHP can't discern the difference between d/m/Y and m/d/Y when just the date
is submitted to strtotime(). Unfortunately, strptime() is not available
until PHP 5.1.0. This patch forces datepickers to change their values to
YYYY-MM-DD upon submission to disambiguate parsing issues.

Fixes #832
Merge pull request #833 from greezybacon/issue/datepicket-date-format

Fix parsing issue in strtotime()

Reviewed-By: Peter Rotich <peter@osticket.com>
Merge remote branch 'upstream/develop' into develop
Fix date formatting on due date (again)

Fixes #834 (again)
Merge remote branch 'upstream-1.7/develop' into develop

Changes up to v1.7.3 released

Conflicts:
	WHATSNEW.md
	css/osticket.css
	scp/css/scp.css
Merge remote branch 'upstream/develop' into develop