Improve default avatar handling in bp_core_fetch_avatar()

Changes to Gravatar over the last few years have placed restrictions on the way
that a default gravatar (to be used as a fallback for when no actual Gravatar
is found) can be served to gravatar.com. In particular, Gravatar cannot cache
and serve default images that are not publicly available (as in dev
environments) or are only available over SSL.

Because BuddyPress's default behavior was to provide a local copy of the
mystery-man avatar, switching to Gravatar's hosted mystery-man solves these
problems for sites that are using the default mystery-man as a fallback image.

This changeset also reconfigures the way that the `BP_AVATAR_DEFAULT` constant
is respected, to ensure that it's possible to override the avatar fallback when
no_grav is set to true.

Fixes #4571

Props r-a-y

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7769 cdf35c40-ae34-48e0-9cc9-0c9da1808c22