Enhance experience when installing and activating a standalone plugin

[mukeshpanchal27]

Summary

Fixes #880

• When Implement migration logic and UI from Performance Lab modules to their standalone plugins #899 will merge we will move the custom JS code in admin/perflab-admin.js file.
• update.js updates plugin activation link in 1 sec so in our custom script we will update it after 1.5 sec.

Checklist

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[felixarntz]

Thanks @mukeshpanchal27, it's great you found a solution for this problem. I left a bit of feedback below, but the overall approach looks solid.

[mukeshpanchal27]

Thank you, @felixarntz, @swissspidy, and @westonruter, for your feedback. I have addressed it, and the PR is now ready for review.

[mukeshpanchal27]

@felixarntz @westonruter, I identified an edge case while installing multiple plugins and try to activate first plugin where we encountered an incorrect selector in event.target.activeElement. Consequently, In b936064 i have updated the selector in the logic, and the issue has been resolved. Could you please review it? Thank you.

[felixarntz]

Great work @mukeshpanchal27!

Only one minor thing that would be good to address, but not a blocker.

Need to merge.

[mukeshpanchal27]

@swissspidy Could you kindly take a moment to review the recent changes?