Add optional email alerts for new questions, answers, and comments

[sdeibel]

Sends an email to a configurable address when new content is posted. Useful for low-traffic sites or monitoring during initial rollout. Hooks into existing askbot signals (new_question_posted, etc.) via AppConfig.ready().

Opt-in: does nothing unless POST_ALERT_EMAIL is set in Django settings.

[evgenyfadeev]

Question: does configuring alerts to be sent for all post not suffice - using an existing feature? You can go to your user profile and max out the email alerts - that way you'll receive email on every post.

[sdeibel]

Hmm, I'm not sure. It might work but in general I never got reliable alerts until adding the above. Sorry, again all these things were added with the sole purpose of getting our site running well enough without needing a lot of attention, and so not all my changes may be appropriate to merge. I figured they might at least contain some useful ideas.