UX for displaying multiple image attachments
This MR shows multiple images in the private conversation where before only one image was shown. It is the bulk of the work to support multiple images. The RecyclerView it uses is recycling views over several distinct messages and can be used to display a preview of images to be attached as well.
For simplicity, multiple images within one message bubble are fixed to two columns. If a message contains an odd number of images, the last images will span both columns and get center cropped.
The corner rounding transformation has been replaced with a new one that allows us to specify a radius for each image's corner individually.
Closes #1473 (closed)