Store SQLCipher decryption key in separate file

First, we write the key a whole lot less. We write it on creation, then
never again.

Second, it's in a file we control very closely. Instead of blindly
regenerating the key if the target file generates an error on read,
we block startup unless the error is 'ENOENT' - the file isn't there
at all.

This still allows for the key.txt file to be deleted or corrupted
somehow, but it should be a lot less common than the high-traffic
config.json used for window location and media permissions.

app/sql_channel.js

@@ -1,5 +1,6 @@
 const electron = require('electron');
 const sql = require('./sql');
+const { remove } = require('./key_management');
 
 const { ipcMain } = electron;
 
@@ -12,16 +13,12 @@ let initialized = false;
 const SQL_CHANNEL_KEY = 'sql-channel';
 const ERASE_SQL_KEY = 'erase-sql-key';
 
-function initialize({ userConfig }) {
+function initialize() {
   if (initialized) {
     throw new Error('sqlChannels: already initialized!');
   }
   initialized = true;
 
-  if (!userConfig) {
-    throw new Error('initialize: userConfig is required!');
-  }
-
   ipcMain.on(SQL_CHANNEL_KEY, async (event, jobId, callName, ...args) => {
     try {
       const fn = sql[callName];
@@ -44,7 +41,7 @@ function initialize({ userConfig }) {
 
   ipcMain.on(ERASE_SQL_KEY, async event => {
     try {
-      userConfig.set('key', null);
+      remove();
       event.sender.send(`${ERASE_SQL_KEY}-done`);
     } catch (error) {
       const errorForDisplay = error && error.stack ? error.stack : error;