Improve layouts of media attachments

This improves the layouts of attachments in several ways:

• Cropped attachments are actually 16:9, instead of the weird ratio Tokodon decided on before. It looks bad when the Tokodon window is big enough. Example of current behavior:

• (Non-expanded) single media attachments can now be configured to either be cropped to 16:9 or displayed at full height.
• Removes the extra margins which should've been on the attachment grid itself, not on individual items. This improves the uniform look of multiple attachments especially.

Related to BUG: 463892

Tasks:

• Aspect ratios are broken when cropped mode is turned on (oops, broke that while preparing for this MR)
• Change it to load "crop media" preference
• Simplify logic, it's horrible
• Edit commit message to include BUG
• The "hide media" button is in the wrong place

Also, viewing Tokodon on a big screen will suck even more with this MR because our layout is causing everything to get stretched (including media), so I'm deliberately scaling images to the full timeline width for now.