Implement basic structure of local database

[redshiftzero]

started in #1, eventually should have the following (ported from freedomofpress/securedrop-workstation#88 (comment)):

sources table

uuid
journalist_designation
is_flagged
last_updated
public_key
interaction_count
is_starred (not using separate table here as we do on the server, there is no reason to)

submissions table

uuid
is_read (the number of submissions that have this designation is basically going to generate the initial “read” icon locally)
filename -> path on the SVS where the doc is stored
is_downloaded (we could also expose this in the sources list in the UI)
size (expose in UI to give the user an idea of how long it will take to download)
source_id -> id in local database

replies table

source_id -> id in local database
journalist_id -> id in local database
filename

journalists table

username -> to begin with, this will just be the current user (when replies are surfaced in the API via freedomofpress/securedrop#3097 then we should store the usernames so it's clear who is talking to who)