Add a handleDbException() method to BaseActivity
This adds a `handleDbException()` method to BaseActivity and a corresponding method for fragments that calls through to the activity.
For now, the method just finishes the activity
and NavDrawerActivity overrides it to do nothing,
and all the error places marked with TODO that finish the activity call the method instead.
That gives us zero functional improvement over the status quo,
but it allows us to change the default behaviour easily,
and then we can start thinking about which cases should have non-default behaviour.
First part of #469
See merge request !469
and a corresponding method for fragments that calls through to the activity.
For now, the method just finishes the activity
and NavDrawerActivity overrides it to do nothing,
and all the error places marked with TODO that finish the activity call the method instead.
That gives us zero functional improvement over the status quo,
but it allows us to change the default behaviour easily,
and then we can start thinking about which cases should have non-default behaviour.
Show open button in private conversation after accepting invitations
To keep the implementation simple, the Open button does appear where the Accept button had been previously.
In order to make the Open button functional, I had to make the `GroupId` of the invitation target available to the UI. Most code in this MR is due to that.

Closes#793
See merge request !457
Ignore QR code results until local QR code is created
* Make scanning only possible after we are "ready" (= our QrCode was created and set).
Signed-off-by: goapunk <noobie@goapunks.net>
Closes#814
See merge request !454
Update all logos
The launcher icons are now in mipmap folders which is the recommended practice, because on a low-res device, the high res drawables are stripped from the app. This is bad when the user decides to show the app icons really big on the launcher and the high-res drawables are not available anymore.
The old launcher icon seemed to have some sort of 3D effect.

Navigation Drawer Old:

Navigation Drawer New:

Closes#828
See merge request !459
Use different notification icons for different features


Closes#795
See merge request !460
Plugins throw exceptions for startup errors
Please review carefully as I don't know much about the code I have touched here.
Closes#285
See merge request !462
Move all unit tests to their modules and remove briar-tests
There are now tests in these modules:
* bramble-api
* bramble-core
* bramble-j2se
* briar-core
* briar-android
In order to run all -core tests together, the following AS run configuration can be used:

Closes#778
See merge request !456
Add sharing information to private group ActionBar subtitle

Closes#646
See merge request !455
Add creator and online information to group member list


Closes#813
See merge request !448