Migrate to SQLCipher for messages/cache

Quite a few other fixes, including:
  - Sending to contact with no avatar yet (not synced from mobile)
  - Left pane doesn't update quickly or at all on new message
  - Left pane doesn't show sent or error status

Also:
 - Contributing.md: Ensure set of linux dev dependencies is complete

app/attachment_channel.js

@@ -0,0 +1,36 @@
+const electron = require('electron');
+const Attachments = require('./attachments');
+const rimraf = require('rimraf');
+
+const { ipcMain } = electron;
+
+module.exports = {
+  initialize,
+};
+
+let initialized = false;
+
+const ERASE_ATTACHMENTS_KEY = 'erase-attachments';
+
+async function initialize({ configDir }) {
+  if (initialized) {
+    throw new Error('initialze: Already initialized!');
+  }
+  initialized = true;
+
+  console.log('Ensure attachments directory exists');
+  await Attachments.ensureDirectory(configDir);
+
+  const attachmentsDir = Attachments.getPath(configDir);
+
+  ipcMain.on(ERASE_ATTACHMENTS_KEY, async event => {
+    try {
+      rimraf.sync(attachmentsDir);
+      event.sender.send(`${ERASE_ATTACHMENTS_KEY}-done`);
+    } catch (error) {
+      const errorForDisplay = error && error.stack ? error.stack : error;
+      console.log(`sql-erase error: ${errorForDisplay}`);
+      event.sender.send(`${ERASE_ATTACHMENTS_KEY}-done`, error);
+    }
+  });
+}