Add a title length warning on the admin post edit page

the new alert looks like:

![screen shot 2014-11-04 at 1 59 48 pm](https://cloud.githubusercontent.com/assets/3730514/4909249/cced2406-6471-11e4-9c0f-d8b883678536.png)
![screen shot 2014-11-04 at 2 00 00 pm](https://cloud.githubusercontent.com/assets/3730514/4909248/cce6e852-6471-11e4-9cbc-91ff4c273f7c.png)

depends on this pull request for our config file: https://github.com/GigaOM/gigaom/pull/5905

(this pull request is essentially the same as [this other one](https://github.com/GigaOM/gigaom-plugins/pull/3137), but 3137 was mistakenly pulled against `gigaom-plugins`.)

see https://github.com/GigaOM/gigaom/issues/5767
shouldn't it be: `if ( 'undefined' === typeof go_title_length_alert ) {`? (declare `var go_title_length_alert` if it's not already defined.)