Remove option to create new issue to resolve all threads in widget

🐙 Note: this is behind a feature flag

What does this MR do and why?

• Removes option to create new issue to resolve all threads in widget
  • Why? Option is still accessible from the threads control in header (and is the only one in our documentation)

• Relabel button to match threads control

Open questions/notes

Could we remove the option to resolve all threads altogether?

• It's an easy way to unblock an MR, but it's far from an ideal flow
• Threads should be resolved thoughtfully, one at a time. If it's not worth fixing or out of scope, create an individual issue for it
• We don't have instrumentation differentiating Resolve all... from Resolve single thread...

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Enable feature flag :hide_create_issue_resolve_all
• Go to MR and open a start a thread or comment on a line of code

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

#394879 (closed)