Remove placeholders and limit SERP description

closes #4112, closes #4113
- Remove meta_* placeholders
- Add &hellip; in SERP preview when desc too long
I think the result of `this.get('meta_title')` should be coerced to a string here.  

If `meta_title` has never been set on the model it will be `null` so if you type something into the meta-title input but then delete it, when it blurs and attempts to save the conditional will be `null === ''` and the save will fire even though it really isn't necessary.