Create a system user for "Imported" translations

[mathjazz]

For translations imported from VCS, the Translation.user field isn't set and the translations appear as "Imported" in the UI. We sometimes need to create in-memory instances of User objects to handle the "Imported" user along with other translation authors more easily:

pontoon/pontoon/base/views.py

Line 415 in 56d243e

u = t.user or User(username="Imported", first_name="Imported", email="imported")

pontoon/pontoon/contributors/utils.py

Line 117 in 56d243e

User(username="Imported", first_name="Imported", email="imported")

Rather than doing that, we should create an actual system_user representing the "Imported" user. It will also save us from things like:

pontoon/pontoon/base/models/user.py

Lines 198 to 199 in 240486d

	if self.username == "Imported":
	return ("", "Imported")