Improve load time perception when generating summary

What does this MR do and why?

1. Add skeleton loader to discussion summary, instead of just the spinner of the button.
2. Fix race condition causing tooltip on the button to stay open after clicking
3. Fix potential race condition causing button to stay stuck in loading state. Can replicate this by not setting up AI backend (skip step 1.1 below and just enable the feature flags). Then clicking the button triggers a mutation, which succeeds and clears the timeout, then the subscription is left waiting forever.

Screenshots or screen recordings

Screen_Recording_2023-08-16_at_11.54.27

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set up discussion summary AI feature
   1. Configure GCP Vertex access
   2. Enable :summarize_comments feature flag
2. Click 'View summary' on an issue page, ideally one where the issue has comments

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.

Related to #414764 (closed)