[PP] module enable controller should block enabling if another enable in progress

Postponed on #3284945: Install endpoints that leverage Package Manager + core APIs →

Problem/Motivation

This would be a feature that the current form submission install can't do but it's possible with this UI. We already have the tempstore registering a module as 'enabling' if there is an enabling in process. We should check for this and return a 500 with a message that another enable is in progress and to try again later.

Steps to reproduce

Proposed resolution

Remaining tasks

• ✅ File an issue about this project
• ☐ Manual Testing
• ☐ Code Review
• ☐ Accessibility Review
• ☐ Automated tests needed/written?