Merge branch '9-conversation-view' into 'master'

Overhauled Conversation View with Message Bubbles

The Conversation View now uses a RecyclerView with conversation bubbles
in alternating colors and vector drawables to indicate message state.

The conversation bubbles have been taken from Telegram
and can be replaced by a UX designer later.
There's also a special bubble for unread messages,
so they are not overlooked when they come in delayed.

This commit also addresses #9, because message text can
now be selected and copied. This is done by using

    android:textIsSelectable="true"

which only works for API level 11 or higher.

If we want copy and paste on lower API levels,
additional measures have to be implemented.

See merge request !36