handle converting pr to/from draft

[matt-fidd]

The assumptions made here are:

• if a PR is a draft, we don't need the [WIP] title prefix
• if a PR is a draft, it will always be considered WIP so needs the WIP label
• if a PR is converted from a draft, it should be considered ready for review, and should update as such
• if a PR is converted back to a draft, it should drop the prefix for consistency and revert back to the WIP label
• if a PR is a draft we don't check for failing CI

[matt-fidd]

I've addressed your comments, not my finest work hardcoding a length in when it used a regex to match the bit I was removing anyway!

I've also extended the regex to match other common patterns that were previously handled in Trafico so as not to break any folk's existing workflow.

[twk3]

Great, thanks @matt-fidd !