UICommon/DiscordPresence: Use GameTDB covers for RPC

[noahpistilli]

Rehash of #10390 but cleaner. Discord allows for using URL's for their RPC images. Documented as of this PR.

The image will be the Dolphin logo if GameTDB does not have a cover for the current game.

Otherwise, the cover of the game will appear in the RPC.

[OatmealDome]

My concerns with this PR:

• We should probably avoid downloading the image if possible, even if it is a relatively small size. We can probably do this with a HEAD request instead of using GET. (Dolphin's code doesn't currently support this, though. Shouldn't be that hard to add.)
• What happens if the network request takes a long time? If the user's network is slow, for example, will UpdateDiscordPresence hang the emulator while it attempts to contact GameTDB?

[OatmealDome]

OK, as we discussed on Discord, I've set up a server-side script that will handle the cover art lookup for us:

https://discord.dolphin-emu.org/cover-art/<region>/<game ID>.png

Here's some example URLs:

• A title that exists: https://discord.dolphin-emu.org/cover-art/US/GAFE01.png
• A title that doesn't exist: https://discord.dolphin-emu.org/cover-art/XY/WXYZ99.png

With this, you should be able to get rid of the network request code.