show 'add code snippet' button on edit page

[maxbeatty]

mostly fixes #247

1. you can now add more code snippets 🎆
2. once you add a code snippet, there's no way to remove it 😬

the broken logic was in the footer due to missing template variables. needed benchmark === false && mainJS === true to get jQuery and the JS that adds button client side

[maxbeatty]

decided to separate these template logic vars from the test defaults. not DRY but easier to reason about

[mathiasbynens]

once you add a code snippet, there's no way to remove it 😬

This was the case in the original version of jsPerf, too. However, if you left a code snippet field + its title empty, it wouldn’t be added to the database.

[maxbeatty]

Since Joi validates the entire form payload it's not easy (if possible) to make rule exceptions. Created #309 to track