Activity: @mentions theme compatibility improvement

Some themes add margin-top to images (e.g. "Eighties"), and we need to remove that here so that the Gravatar lines up with the text.

See #3278

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9006 cdf35c40-ae34-48e0-9cc9-0c9da1808c22