jobqueue: Mark JobSpecification as newable

Why:

• JobSpecification is the preferred way to enqueue jobs without instantiating a full Job subclass instance, and the only way to enqueue a job in the context of a foreign wiki.
• As such, it's used by several extensions. However, it's not explicitly marked as newable.

What:

• Mark JobSpecification as newable.

Change-Id: I3dca96857c875da1ee6f0f6054a12aa6ec276697