Fix editing posts.

AwfulForm changed to include submit parameters by default. If the Forums see a
"preview" parameter, it will always preview a post, even if the other submit
parameter is also set.
Add -preview... methods to AwfulForumsClient.
Preview posts.
Formatting.
Show embedded images when previewing posts.

We use yet another custom URL protocol to get the images into a UIWebView. The
only other bit of trickery is that the Forums refuses to parse [img] BBcode
unless the URL resembles an HTTP URL. To work around this, we prefix our custom
URLs with http:// then remove that prefix after fetching the preview.