Implement admin pointer to indicate to the user they need to migrate modules to their standalone plugins

[mukeshpanchal27]

Summary

Fixes #653

Prior to merging this PR, we'll need to ensure that #899 is merged.

Checklist

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[felixarntz]

Thanks @mukeshpanchal27. This looks good, though I left a few suggestions for improving the new function which is a bit too specifically tied to a UI consideration only relevant in one place.

[felixarntz]

Another thought on this (which if we go for it, we could implement in a separate PR): We may want to "un-dismiss" the migration pointer when the user activates a module? This would ensure that, if the user brings the site back into a "problematic" state after initially having fixed the problem, the pointer shows again. I think that would be helpful, especially if the logic here remains in the plugin indefinitely (see related #904 (comment)). We could just always remove the pointer from being dismissed whenever a module is activated, as the pointer would only actually show anyway if relevant for the site's current state.

WDYT @mukeshpanchal27 @swissspidy?

[mukeshpanchal27]

Another thought on this (which if we go for it, we could implement in a separate PR): We may want to "un-dismiss" the migration pointer when the user activates a module?

Yes. IMO we should do that.

[felixarntz]

Thanks @mukeshpanchal27, looks great!

[felixarntz]

Another thought on this (which if we go for it, we could implement in a separate PR): We may want to "un-dismiss" the migration pointer when the user activates a module?

Yes. IMO we should do that.

Thanks @mukeshpanchal27. Can you open a follow up PR for this when you get a chance? Not a blocker for the release, but a nice to have enhancement.