UI: Thread ID Input

[scragly]

An extension to feature request #59

While thread_id is now supported as a URL parameter, it was recommended to put an individual Thread ID input field to avoid the need for users to manually append ?thread_id=0000000000000000 to their webhook URLs, and to help make it obvious to users that threads are now supported.

This is it's own issue due to the original issue hopefully being resolved after PR #64 is merged.