refactor Controller.get_source_count() into securedrop_client.storage

[cfm]

securedrop-client/client/securedrop_client/logic.py

Lines 1219 to 1223 in 0f02485

	def get_source_count(self) -> int:
	"""
	Return total sources in local storage.
	"""
	return self.session.query(db.Source).count()

On second review I realize I was thinking of this in the wrong context: I thought this method was a function in securedrop_client.storage. Strictly speaking, the Controller shouldn't make a session.query() directly. But this is just a matter of organization, not threading, so let's leave it for now.

Originally posted by @cfm in #2300 (comment)