Plugin directory: Prevent duplicated plugin submissions

[frantorres]

Sometimes plugin authors inadvertently submit the same plugin multiple times; when spoken to, they explain that they thought the process had stalled and had tried to resubmit the form.

This adds two methods to prevent duplicated plugin submissions.

• Frontend: Prevents click again in the submission button and changes the text to "Uploading…"
• Backend: Creates an "upload lock" for that user and that plugin (0 when new) that gets removed after the upload has been processed (if for some reason the process fails, it is saved as wp_cache_add for 5 minutes so they can try again after waiting a bit)

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props frantorres, timse201.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[timse201]

Plugin translations are imported automatically when it lands in trac. The best solution would be to allow imports only for PTEs. This prevents such activity on a permanent basis without disrupting legitimate activities.