UI improvements for media descriptions

[cadadr]

Hey, thanks for this great app, my favourite fedi client by far!

I want to make a couple suggestions/feature requests to improve the UI for typing up media descriptions.

Currently the relevant popup is just a single line of text.

I think two changes would improve this a lot (and this is also how Tusky does it):

1. Adding a small, zoomable preview of the media in the media description popup.

2. Replacing the single line media input in the said popup with a multiline one, like a in HTML.