Fix for issue 5090

https://github.com/diaspora/diaspora/issues/5090

Changing error message wording and providing more information to user when status message is too long. I can add a test if you'd like, but none existed before. All tests pass. I'm not a huge fan of the controller change (@status_message.errors.messages[:text].join('. ')) as it looks clunky and the model change (adding a proc to the validates message) looks odd, but that seems to be the way to add the current message length.

I think this also will fix https://github.com/diaspora/diaspora/pull/5099 as it appears to be similar if not identical
Regarding the magic 'messages' method, I think I've just used it in the past and it still works so when I typed it I didn't think anything of it. I don't see it in the documentation though, so that's fairly interesting.

The full_messages or full_messages_for(:text) produce this as an error message, "Text Please make your status message fewer than 65535 characters. Right now it is 93141 characters" It's all good except from the leading "Text." I could rewrite the error message to read something like "is too long at %{count} characters. Right now it is %{current_length} characters." Then the leading "Text" fits in, although we lose the (more correct) reference to status message.

Whichever you prefer.