No longer trigger sync when files are missing

[sssoleileraaa]

Description

Remove the areas of the code that sync with the server when a file is unexpectedly missing, e.g.

securedrop-client/securedrop_client/logic.py

Lines 647 to 649 in 7dc80f8

	if not self.downloaded_file_exists(file.uuid):
	self.sync_api()
	return

Instead of waiting until a sync to update missing files so that they can be redownloaded by a user, do this immediately:

securedrop-client/securedrop_client/logic.py

Line 425 in 7e3cfca

storage.update_missing_files(self.data_dir, self.session)

[eloquence]

Per discussion at sprint planning today, this is blocked by completion of #473; it may make sense to wait until #288 is completed, as well.

[ntoll]

Hi, I'm blocked on starting this until my outstanding PR's relating to the two issues mentioned by @eloquence are merged. Happy to pick this up once they're merged.