Unselected providers should disable webmentions too

[snarfed]

One awkward bit of the current webmention/synd links outbound syndication setup is, if I uncheck the Bridgy Fed syndication checkbox, but the post links to Bridgy Fed in its content, the webmention plugin still sends BF a webmention and the post still gets syndicated. Could we prevent that? I know that's technically not in the synd links plugin, but does the wm plugin have hooks that synd links could use to suppress that specific outbound webmention?