Allow user to force login required.

Use case: I just want to enable http auth for feed readers. The feeds are also public, but if people want to have all the content they have access to with their user account they need to log in, and with an automated reader http is easiest.

Issue: Thunderbird and Firefox seem not to send the authentication headers if they are not required. Firefox pops up a dialogue. Thunderbird (well Lightning in this case - ical feeds in thunderbird calendar) just ignores them silently.

Solution: Allow an additional GET query login=force which makes it required to authenticate. I just added this as a line to the _securesite_forced() function. See attached patch.