bug fix: redirect to the protected page after login

add a 'next' attribute in login html page.

if "next" is not None, means there is a html protected page user
want to access. after authentication success, redirect to it.

if "next" is None, then redirect to the last page recorde in th the
cookie.

Test this case:
1. input a protected page URL in the browser.
such as:
https://localhost:8001/spice.html?port=64667&listen=localhost&token=vmname&encrypt=1

2. input a wrong password or username:
it should report:
"The username or password you entered is incorrect. Please try again."

3. input the right password and username:
It should redirect to the former page.
https://localhost:8001/spice.html?port=64667&listen=localhost&token=vmname&encrypt=1

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
remove kimchisession hook and add the same logic to root.get

kimchisession hook is heavy overhead.

root.get just return the kimchi-ui.html, it is not a restful API.

consider root.get is the only entry of UI.

So move the logic code to root.get.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
update case for root.get redirection

update test case.

when the user access kimchi firt time, root.get will redirect to
login.html instead of kimchi-ui.html.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>