bmcweb: support next= url for forwarding login

Related to patchset here.  This patchset implements the bmc side of
redirecting the users URL after login correctly.

https://gerrit.openbmc-project.xyz/#/c/openbmc/phosphor-webui/+/15925



Change-Id: Idf42f4cecd29fb0c6088721eb5e10fb39e1168e7
Signed-off-by: Ed Tanous <ed.tanous@intel.com>