After journalist designation is changed, we must rename files that have already been decrypted

[redshiftzero]

Followup to #250, discovered during review of #277

Steps to Reproduce

0. Submit file as a source
1. Log into client, download the file. e.g. it will be saved in your client data dir as 2-ducal_hellhound-doc when successfully decrypted
2. Cycle the journalist designation for that source, e.g. to translucent intrigue
3. Refresh the client
4. Source will appear with the new codename
5. Try to open the file you downloaded: log line like the following will appear:

2019-03-26 07:56:13,591 - securedrop_client.logic:578(on_file_open) INFO: Opening file "/var/folders/gv/2r10zvfj303fyd5_6yn_p0b80000gn/T/tmp.USqujvA3/data/2-translucent_intrigue-doc".

Expected behavior

/var/folders/gv/2r10zvfj303fyd5_6yn_p0b80000gn/T/tmp.USqujvA3/data/2-translucent_intrigue-doc exists on disk and thus can be opened

Actual behavior

Downloaded/decrypted file is still called /var/folders/gv/2r10zvfj303fyd5_6yn_p0b80000gn/T/tmp.USqujvA3/data/2-ducal_hellhound-doc

Comments

When a journalist designation changes, we should also rename files that have been decrypted, i.e. those that no longer end in .gpg

[redshiftzero]

this feature will be gone on server side as of 1.2.1 so closing (ref freedomofpress/securedrop#4304)