Offline support

[maxking]

Currently, the entire functionality of the system relies on online API calls to Hyperkitty. This can cause un-necessary load on servers for re-reading same threads/emails and not to mention. Ofcourse, it removes the ability to use this package to read archives when offline.

In order to support offline support, we need to accomplish the following things:

• Define some local storage method, it can be files or sqlite databases. Files might be easier to manage.
• Write responses from API to local storage
• Render all views based on local storage.

Each of the above will also have some sub-tasks.