Allow syncing in the background

[vickychijwani]

Until this is implemented, there will always exist a high possibility of bugs like this:

1. User makes significant edits to a post
2. A phone call comes in, app goes to background, edits are saved but not synced to the server
3. User does not open the app for several days
4. In that period, someone else (or this same user) makes some edits to the same post through the web interface -- at this point the user may notice their several days old edits have not appeared yet!
5. Now the user opens the app and a sync occurs -- this will end up overwriting the remote copy with edits in step 4 with the old edits!

Concerns to take care of:

• [ ] Ensure background service starts automatically on boot using ACTION_BOOT_COMPLETED

[vickychijwani]

This sample offline architecture may help (presented at the Android Dev Summit architecture talk): https://github.com/yigit/dev-summit-architecture-demo

[vickychijwani]

Removed from 1.0 as this is a pretty big change.

[vickychijwani]

GCM Network Manager looks like the best way to do this: it's convenient, flexible and conserves battery. Here's a great overview: https://www.bignerdranch.com/blog/optimize-battery-life-with-androids-gcm-network-manager/

It should be noted that with GCM moving over to Firebase, the GCM Network Manager will eventually be superseded by Firebase Job Dispatcher.

UPDATE: Yigit Boyar's Priority JobQueue library might also be helpful.