Add confirmation modal when user wants to import into a personal namespace

Problem

This relates to #525342 (closed)

The problem is that users lack a UI for reassignment, making the mapping of contributions to the Import User pointless if they cannot carry out the task. Additionally, since users won't perform the reassignment, the placeholder user references will remain, leading to unnecessary records in the database.

Proposal

To raise awareness of this problem and explain the side effects to users when migrating projects into a personal namespace, we should inform the prior to performing the migration. Thus,wWhen a imports projects from either GitHub, BitBucket Server or Gitea and selects a personal namespace on the destination instance, we should display a confirmation modal:

.

The modal should have the following copy:

Are you sure you want to import the project to a personal namespace?
When you import to a personal namespace, all contributions are assigned to a
single non-functional user called `Import User` and they cannot be reassigned.
To map contributions to actual users, import the project to a group instead.