add content snippet to notification messages for posts and replies

...and other tweaks.

example notification messages:

[mention] http://www.kryogenix.org/days/2014/11/30/vouching-for-webmentions-hashing-for-vouches/ linked to http://indiewebcamp.com/Webmention (webmention)
[mention] The Dod liked a post linking to https://snarfed.org/2014-11-24_kyle-mahan-let-them-eat-dogfood-httpgogs-io-2 (https://dubiousdod.org/indie/2014/11/like-of-ryan-barrett-kyle-mahan-let-them-eat-dogfood)
[mention] Tantek Çelik posted 'What apps have #oauth access to your accounts? I created this page to help indiewebcamp.com/FreeMyOAuth Inspired by freemydata.co, expanded ...' linking to http://indiewebcamp.com/FreeMyOAuth (http://tantek.com/2014/324/t1/apps-oauth-access-accounts-freemyoauth)
[mention] Ryan Barrett posted 'One big IndieWeb raison d’être is using your own web site to reply, like, repost, and RSVP to posts and events. You do this by annotating li...' linking to http://indiewebcamp.com/ (https://snarfed.org/indieweb-press-this-bookmarklets-for-wordpress)
[bridgy] Devon LaHar commented 'OK, scary....' on a post linking to https://snarfed.org/2014-11-25_11746 (https://facebook.com/212038/posts/10101380661450183?comment_id=10101386962532763)