Show fork suggestion when user cannot perform in add-to-tree dropdown
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## Proposal
@alyssatrinh **directory page actions** I also noticed that when users who don't have permission to modify a directory try to add a `New file` / `Upload file` / `New directory`, no messaging or alert is shown before the fork is created for users. This can be quite jarring, as users may go from trying to add something to a directory, to a fork being created (and potentially taking a long time) without any input or confirmation. We don't provide users a way to opt out of creating a fork from the directory pages. A modal may help in this situation as well.
| Directory page: fork is created automatically | File page: users are shown message before forking |
|-----------------------------------------------|---------------------------------------------------|
|  |  |
I would suggest going with a modal instead of an alert for the directory and file page actions that would prompt a user to fork a repo:
* Directory page
* `New file`
* `Upload file`
* `New directory`
* File page
* `Replace file`
* `Delete`
* `Edit`
I also suggest simplifying the text so that we can use the same modal on the directory and file page:
* **Fork to make changes**
* You're not allowed to make changes to this project directly. Create a fork to make changes and submit a merge request.
* `Cancel` `Fork`
## Scope
Include ForkSuggestionModal for the actions in [AddToTree](https://gitlab.com/gitlab-org/gitlab/-/blob/d3cf7b34544d74e8bf25d99dd7a7fa3d5db4e548/app/assets/javascripts/repository/components/header_area/add_to_tree.vue) that result in forking the project before completing the action.
issue