Add DownloadJob

[sssoleileraaa]

Description

Closes #406

This is part of breaking down a large PR #409

This and following smaller refactors are a way to help us avoid bugs around file management when we download and decrypt, which is tricky.

Summary of changes

• Moved call_api, _download, and _check_file_integrity into a new base class called DownloadJob from FileDownloadJob and MessageDownloadJob. _decrypt will also be moved into DownloadJob from its derived classes in a followup PR.

• This also introduces new behavior: checking file integrity of everything that gets downloaded. So now message downloads also get checked.

Test Plan

No need to test in Qubes, but always feel free. This PR doesn't modify how we decrypt or download, and the extra file integrity checks is not qubes-specific.

1. Send messages and files as a source
2. Refresh on the client to see messages downloaded and decrytped. Click on files to trigger a download and decrypt. When you see "Open" next to the filename you know it's decrypted.
3. [optional] query the db to make sure statuses look correct

[redshiftzero]

Confirming:

1. This closes refactor: MessageDownloadJob and FileDownloadJob should inherit from new class DownloadJob #406 correct? If so let's add to the PR description such that the GitHub autoclose will happen when this PR is merged
2. I think there should just be one commit here (rebased on latest master after merge of Version 0.0.8 #428 fixing CI). It looks like a merge commit made its way onto this branch

[redshiftzero]

Base branch here should be master no?

[redshiftzero]

took a look at this - I wasn't able to download files or messages on these latest changes, see comments inline

[sssoleileraaa]

alembic tests are failing - will look into this tomorrow

[redshiftzero]

heads up that the alembic tests fail here because the removal of constraints from existing tables is not supported in sqlite due to incomplete ALTER table support. However, we can still do these constraint removals via the following steps:

1. Rename the table my_table to be modified to e.g. my_table_tmp
2. Create a new table my_table with the constraints applied that we want
3. Move each row from my_table_tmp into the new table (my_table)
4. Once all rows are migrated, drop my_table_tmp

You can follow this example here which is from when I added the new journalist UUID column with a unique constraint on the server-side (also uses sqlite)

[sssoleileraaa]

thanks @redshiftzero! worked like a charm 🍀

[redshiftzero]

just tested this and read through the updated diff, all my comments were addressed, LGTM 🚀

[redshiftzero]

note that we need packaging change freedomofpress/securedrop-builder#59 to land also, let's wait until that is approved before merging this (good to get in this practice as we always want to leave master in a state ready for nightly builds)