handle big events better

[snarfed]

they often put us over the memory limit right now. very few events have original post links that accept webmentions, so the easiest thing to do would be to check that first and short circuit out.

[snarfed]

happened again with https://www.facebook.com/events/1394478120817149/ . i should probably do something about this.

[snarfed]

Facebook support is dead (#817, #826), so I'm closing this as obsolete. feel free to reopen if you have any concerns!