Check whether File#listFiles() returns null
The docs say this can happen if there's an I/O error. Also fixed a throw-in-finally-block warning and renamed an IoUtils method to make its contract clearer.
See merge request !328
Remove base32 TODOs, fix a potential NPE
I thought we'd be able to get higher data density in QR codes by using base32 instead of base64, allowing the QR code to use alphanumeric mode instead of byte mode. But I tried it, and although the QR code does use alphanumeric mode, it comes out at exactly the same size (see the 346-use-base32-for-qr-codes branch). So this MR removes the TODOs and fixes a potential NPE I spotted while working on the other branch.
See merge request !327
Recreate cache directory after deleting app data
This fixes a warning on the Moto G:
```
E/libEGL: error creating cache file /data/data/org.briarproject/cache/com.android.opengl.shaders_cache: No such file or directory (2)
```
@grote might possibly be related to hardware rendering issues.
See merge request !326
When the user enters a private conversation after adding her second
contact, an onboarding screen will be shown highlighting the possibility
of introducing the contacts to each other.
Show Blog Posts always in the same design
This also adds comments to the blog post detail screen that were previously missing.
It re-uses the existing `BlogPostViewHolder` and its `bind()` methods effectively reducing the amount of code necessary.
Closes#636
See merge request !307
Attach optional messages to invitations in Conversation view

Closes#359
See merge request !299
Comments and reblogs need to depend on the post they refer to.
Since message dependencies are limited to one group,
the post and also the comments need to be wrapped
when commented on or reblogged to another blog.
For this reason, in addition to comments, two new wrapping message types
are introduced. They retain all data of the original messages and allow
for reconstruction and signature verification.
This commit breaks backwards compatibility with old blog posts.
It removes the content type, title and parent ID from the post
message structure.
Blog controller thread safety
This patch removes the mutable list of posts from the blog controller to make it thread-safe, and adds a cache of message bodies to speed up reloads.
Closes#555.
See merge request !276