Use HTTPS link for GitHub ribbon image
Use HTTPS link for GitHub ribbon image

This prevents the mixed content warning when the docs are hosted on an
HTTPS site.

Also, use an HTTPS link for the GitHub project.

nfvs/flask-login,nfvs/flask-login,maxcountryman/flask-login,HoustonFortney/flask-login,neilsh/flask-login,neilsh/flask-login,HoustonFortney/flask-login,alanhamlett/flask-login