Finish settings/inbox page refactoring

[rrrooommmaaa]

1. Render attachment loading errors

Originally posted by @rrrooommmaaa in #5069 (comment)

2. Currently, settings/inbox rendering differs from webmail in a way that it first accumulates HTML strings and then adds them to the DOM. So if this is called by MessageRenderer.processAttachment, iframe#${frameId} may not yet exist in the DOM.
   We should refactor inbox to act more like webmail, that is, first adding some placeholders to the page, and then modifying/appending to them.

Originally posted by @rrrooommmaaa in #5069 (comment)