Add sharing via LinkedIn Messages

Closes #744

Pretty straightforward, just use
- to: user@linkedin
- title: Subject line
- body: Contents

Recipients must be referenced by their per-application generated ID; not their
numeric LinkedIn ID, nor the name in the custom URL. Will document.

Since LinkedIn response with absolutely nothing besides an `HTTP 201`, I've
inserted a friendly `success: true` to let developers know all is well.