In-app media previews

`MediaPreviewActivity`, currently only supporting images, will be used to show attachments in-app in order to avoid the contents being exposed unencrypted.

Refactored saving attachments logic into a redone `SaveAttachmentTask` to be used in more than one place.
Actually, I just made this private and put all the helper functions for media slides in this class. Makes more sense to me and saves code.