Create and use import_bot in BareRepositoryImport

BareRepositoryImport assigns the first administrator of the database as the creator of the groups/projects imported using the tool.

However, we should use an internal user (bots)

Solution breakdown

1. Create a new bot (import_bot), like it was done in !30738 (merged)
2. Add the new bot to the documentation
3. Update the importer to use the import_bot instead of the admin user