Bug 1774334: Correcting redirect on login for site in subdirectory

The fix in Bug 1774309 fails to work correctly when the site
is in a subdirectory of the apache root.

To test:
1) Adjust your site's apache file and set the DocumentRoot to be
parent of htdocs
2) Change the 'wwwroot' db value for site to include the 'htdocs/' bit
3) Restart the apache server
4) Go to site, adding 'htdocs/' to url
5) Try logging in

before patch: end up not found page with url containing two 'htdocs/'

after patch: end up at correct place

behatnotneeded

Change-Id: Ic5e1de2c5edaf96bbdd9b2403f0f65e45a4b80c0
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
(cherry picked from commit d9ce7d92)