Implement offline resource caching

[taoeffect]

Problem

If we are offline we still want to be able to load the app, use it, and display attachments, avatars, etc.

Solution

There are several possible solutions:

• We could use IndexedDB as a cache for files [1] [2], however this doesn't help us load the entire site if we're offline, and checking the quota seems difficult.
• We could use Service Workers (#87), which are designed to for this and have cache API. Note they require HTTPS so we may also need to use something like indie web server instead of grunt-contrib-connect...

I'm leaning toward Service Workers. However, not having full control of the cache is a potential issue in terms of decentralization of data...

Also, if we are trying to support the website working in the browser (i.e. outside of Electron/desktop app), then we need to make sure we can handle recovering from the cache being deleted out from under us.

See also:

• https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API/Using_Service_Workers
• https://developer.mozilla.org/en-US/docs/Web/API/IndexedDB_API/Browser_storage_limits_and_eviction_criteria
• https://developers.google.com/web/updates/2016/06/persistent-storage

EDIT: related: #601 — Handle large files differently

[taoeffect]

This is now almost finished thanks to #593, however we still need to properly handle running out of space.