Long post titles can lead to invalid hostnames in Netlify deploy previews

**- Do you want to request a *feature* or report a *bug*?**

*bug*

**- What is the current behavior?**

*CMS entries for posts with long titles can create branch names too long for deploy previews via Netlify's standard branch deploy naming scheme. DNS hostnames can be a maximum of 64 characters per "segment" so "a-very-long-blog-post-title-because-i-am-a-really-really-wordy-dude--sitename.netlify.com" will not be a usable hostname and thus the URL will fail to load with various/unspecified bad behavior.*

**- If the current behavior is a bug, please provide the steps to reproduce.**

*create a post with a title longer than 64 characters and attempt to load the branch deploy from the deploys tab in Netlify's UI*

**- What is the expected behavior?**

*Branch deploys would be loadable by their normal hostname, branchname--sitename.netlify.com . Since there is no way for the CMS to know what "sitename" is there we should probably pick a short-ish cut-off like 40 characters - it won't solve EVERY situation but it will do better than the current which allows you to create an impossible branchname of >64 characters which will NEVER load properly (even for a sitename like 'a.netlify.com').*

**- Please mention your node.js, and operating system version.**

*Netlify default build (node v6.11.2 (npm v3.10.10)/ubuntu 14.04 x86_64) && CMS version 0.4.4*