Endpoint description should be optional

- Redefines `HttpPartConfig#description` as `Option[String]` from `String`
- Removes `validate[required]` from description form-control in Part edit UI
- Adds "validate[required]" to Threadpool selector in Endpoint Edit UI

Brought up by @cb372's [comment](https://github.com/m3dev/octoparts/pull/39#issuecomment-58153500) on #39