Prevent lost updates by using ETags for notes

[korelstar]

Finally, I've implemented ETags for notes / If-Match HTTP header for the notes API. This fixes #56.

Includes:

• API: expose the note's ETag and check note's ETag on update request (HTTP If-Match / 412) => new minor API version 1.2
• documentation for API changes
• Test cases
• web-frontend: check for update conflicts and provide solutions

@stefan-niedermann @phedlund : Please have a look if you see any problems with these changes. They are downwards-compatible, so you don't need to change your implementation. However, it is strongly recommended in order to prevent lost updates. If you implement it, your implementation is still compatible with older (server) Notes app versions, since they just ignore the If-Match header.

[korelstar]

Any feedback on this? Any plans to implement this in your apps @stefan-niedermann @phedlund?

Also cc @davidbannon since it is related to #621 and #627.

[stefan-niedermann]

Yes, i would like to implement support for it, though i won't be able to make it in the next few weeks due to some other responsibilities and tasks i have to do. I also will probably need some help for the concrete implementation.

[korelstar]

Okay that's fine. I think it would be good to finish your Room migration before starting with this, since this may require some DB changes. Of course, I can give some advises/help then. :-)