Configure http client

- HTTP client properties are now injected from the HTTP part config. `KeyedResourcePool` now distinguishes the 'key' and the source object from which to create a key and a resource.
- to this effect, HTTP part config has 4 new fields (pool size, connection timeout, read timeout, and default encoding)
- those 4 fields are configurable in admin screens

Also :
- split `PartData` form which was getting too big to add new fields
- `HystrixConfig.timeoutInMs` is now a duration (for consistency). However, it seems unnecessary to change the field name (and so the column name) at this point.
- now outputting form global errors without overriding the previous error messages in `AdminController`
- added debug of field validation errors in `AdminController`. Those should not happen if client-side validation is done properly.
`getMessage` from an exception could be null. Maybe revert back to errorMsg being an Option and wrap that in an `Option.apply` call.