Convert Twitter from using item->title to item->content

[MitchellMcKenna]

Currently the twitter plugin sets the item content to empty, we should stop doing that. Twitter themes should use the content as the tweet, not the title tag.

Since previously all content was set to empty, a migration will have to be created which copies the title into the content where content is empty for twitter items. Relies on completion of #19.

There shouldn't be html in the title. Once this ticket has been completed, title should no longer be run through _autolink() function.

Once complete _autolink() function can be removed since content no longer uses _autolink() either since 2958721.