Use contact ID rather than messaging group ID to identify conversation

We originally used the private messaging group ID to identify the private conversation, but now that the conversation includes messages from multiple clients it's more appropriate to use the contact ID.

This refactoring isn't urgent - I've had the branch lying around for a while, but I'm putting it up for review because #734 will touch some of the same code.

See merge request !386

Allow messages to be deleted in delivery hook

Closes #643

See merge request !385

Add unit tests for KeyManagerImpl

This also creates a `TransportKeyManager` interface and a factory for that to be able to test things separately.

Closes #205

See merge request !380

interface and a factory for that.

Let only the creator invite new members to private groups

A little bug I noticed when reviewing the implementation of the invitation protocol.

See merge request !383

Remove new member announcement and add signature to join message



See merge request !384

Replace transaction.setComplete() by database.commitTransaction()

Closes #674

See merge request !374

Improve QR code workflow slightly

* Improve wording so contacts know they need meet up to scan
  and scan each other's codes
* Use consistent progress bar styles

Closes #348

See merge request !381

* Improve wording so contacts know they need meet up to scan
  and scan each other's codes
* Use consistent progress bar styles

Add Unit Tests for ClientHelper

Closes #724

See merge request !379

Don't throw IllegalStateException if BDF input is incomplete

Closes #731

See merge request !378

Limit the depth of nested BDF structures

Closes #518

See merge request !375

Cache the local author and load before the db latch is released

Closes #427, #588 

See merge request !354

added a cache to the IdentityManager, changed its signature, modified when and where the author is stored

made the author creation single-threaded again in the LifecycleManager, removed redundant code

Add Unit tests for ContactManager

Closes #723

See merge request !376

Implement UX design for inviting new members to a group

Closes #722

See merge request !373

Implement UX for viewing the membership of a private group

This MR is the second and last MR to address #678. The first part is in !377.

![device-2016-10-26-112000](/uploads/8cbdee65c123a6d5329e208d9983d0b0/device-2016-10-26-112000.png)

Closes #678

See merge request !364

Remove content-type and parentId from private messages

and turn them into a regular string.

Closes #600

See merge request !372

Implement UX for when a group has been dissolved

This MR is based on !367.

Closes #672

See merge request !369

Implement UX for leaving a group

This MR also includes the creator's part of the UX for dissolving a group since it is almost the same.

![device-2016-10-26-185615](/uploads/8689a1ee103fcee23105a469b37c59de/device-2016-10-26-185615.png)

Closes #671

See merge request !367

Update blog backend to match current usage

Closes #700

See merge request !371