Inline images are not using thumbnails

[neilalexander]

Currently, when an m.room.message of type m.image is rendered, the full resolution file is downloaded. This means that potentially huge images are scaled down for drawing inline, which causes Seaglass to feel sluggish when resizing/scrolling. (The same was true before the avatars were switched to thumbnails.)

Need to switch to thumbnails for this and only download the full resolution image when actually needed.

• [x] Download and display thumbnail
• [x] Download and Quick Look full image when clicked
• [ ] Show download progress when downloading full image

[neilalexander]

This still needs some visual feedback that a download is taking place, and some checks to make sure a download isn't already in progress, but it is a start.