added possibility to add branding to seahub

I added three variables to the settings file.

LOGO_PATH: A path to an image file which will then be used instead of the seafile logo
LOGO_URL: A url that will be used instead of the seafile.com link in the logo
BRANDING_CSS: the admin can add custom css, so that seafile matches the branding/corporate design of the company. It is simply imported after the rest of the seafile css, and thus overwriting the set values.

This was requested for the installation at my workplace and I think this would be a usefull extension for other users/admins too.