Don't query gravatar.com if allowGravatar is false

One reason Gravatars may be turned off is so an organization can avoid
leaking e-mail hashes to gravatar.com.
Simple local avatar support

If the property web.allowGravater is true and web.localAvatarUrl is not empty,
then avatar images are not requested from gravatar.com, but from another
service defined by the web.localAvatarUrl property.  The image is
requested via HTTP GET from the address defined by:

String.format("%s%s-%d.jpg", web.localAvatarUrl, email_username, width)

where email_username is everything to the left of the '@' sign, if
present, in the supplied username argument.  If no '@' sign is present,
the entire username is used.  It appears that the Gitblit code always
requests with the user's e-mail address.