skip target urls over 500 chars

fixes #273. this is really rare, and i'm lazy, so i'm going with the easiest fix. :P
add "select/deselect all" checkbox to /admin/responses
minor: comment why we don't check blog links as webmention targets until propagate
support facebook username as well as id in user urls (e.g. brid.gy/facebook/snarfed.org)
ereporter exception
Revert "don't allow blog webmentions against tumblr sources that haven't set up disqus yet"

This reverts commit 43d0a870867265e92bf5d70ed4a9ad15fa7ead86.

going to do this a different way due to #277
useful error message on incoming webmention to tumblr account without disqus shortname

fixes #277
add FAQ for twitter publish 401 errors. fixes #268
ereporter exception
new FAQ: how to use a short link when publishing

thanks for the nudge, @dissolve!
when signing up an existing twitter account for listen, disable publish

...because we use x_auth_access_type=read to request just read permissions for listen, and if we have an existing read/write access token, that *demotes* it to a read-only token! twitter, you so crazy.

thanks to @gRegorLove for his help debugging this! and cc @kylewm
publish: better error messages for when source is disabled or missing publish feature

for #279
tumblr: extend regexp for finding disqus shortname

fixes #278
publish: apply interpret_http_exception() to create() exceptions so we report HTTP response body

for #279
preview/publish: always use url provided by user, in case it's a short url that redirects

cc @dissolve
bump up from 5 to 10 log lines to search for entity key
jkphl fixed his cron job!
show retry buttons on targets we gave up on as well as targets we're still retrying
allow retrying completed responses with failed targets
publish: handle multiple accounts with same domain. use first one that's enabled.