Send inbox notification when an ability is earned/granted

[cellio]

Currently, abilities are granted when the criteria are met, via a daily script, but the changes are silent -- you have to notice that you now have an ability. New abilities are a big deal; let's at least send the user an inbox notification. (Broader publicity should be considered as part of #1360.)

This raises the question of whether, for transparency, we should also send a notification if an ability is revoked or suspended. I think we should, though we currently have no way to send an accompanying message as part of the suspension/revocation. (Mods can of course send a separate mod message.) We don't want to force people to ask publicly on meta about what should be private matters, but we also don't want to surprise them when they try to do something they thought they could do but can't any more. The new mod tools will give us private comment threads, so maybe we should wait for that to do this part?