macOS: support retina display

[PhilippvK]

When I am using the highDPI display setting on my Macbook, Icons and Profile pictures look blurry compared to the text and menus.

[CalmBit]

Is there any chance that converting these images to SVGs would help fix this?

[muesli]

Yeah, we should switch to using SVGs where it's reasonable. The graphics displayed in the message panes may have to remain PNG based though, as it's simply a lot faster to render. We can ship with higher resolution images for HiDPI displays though.

[wezm]

Just a note that this isn't just macOS as per the issue title. I see the same thing on Linux.