Responding to pull request review feedback

- messages.getQuoteObjectUrl: early return
- backup.js: explaining variables for long if statement
- types/messages.js: Log if thumbnail has neither data nor path
- sendmessage.js:
  - remove extraneous logging
  - fix indentation
  - upload attachments and thumbnails in parallel
- preload: don't load fs for tests, just fse
- _conversation.scss: split two selectors into two lines, 0px -> 0
- backup_test.js: use fse.existsSync and comment twoSlashes regex
- network_tests_view_test.js: Comment duplicate assignment to window.getSocketStatus

preload.js

@@ -209,7 +209,6 @@ require('./js/spell_check');
 if (window.config.environment === 'test') {
   /* eslint-disable global-require, import/no-extraneous-dependencies */
   window.test = {
-    fs: require('fs'),
     glob: require('glob'),
     fse: require('fs-extra'),
     tmp: require('tmp'),