media-sharing: Display images in chat, Fix chat colour issues

Images are now displayed in the chat. For the case that they are available locally (currently only possible by sending them), the images are loaded directly, otherwise they're loaded via. HTTP. Of course that still needs to be changed since those images are not cached and thus are downloaded every time.

Fixes the mediaLocation attribute in the database of messages; it's now the full file path, before it was accidently set to the folder's path.

The colour for timestamp labels of incoming messages wasn't clearly visible in some configurations, it is now always a bit darker (1.3 times) and well visible in all common styles.