[Admin] add a simple [p]editrole icon command

[japandotorg]

Description of the changes

Adds a simple [p]role icon command as suggested in the #feature-ideas channel

The approach might not be well thought right now, but I'm open to suggestions on how to improve it, etc etc.

Have the changes in this PR been tested?

No

[Flame442]

You should follow similar logic as [p]set bot avatar follows for processing the str link input, namely filtering out <link> syntax and being slightly more verbose as to what exception the get raised.

Red-DiscordBot/redbot/core/core_commands.py

Lines 2867 to 2878 in ca84cc9

	elif url is not None:
	if url.startswith("<") and url.endswith(">"):
	url = url[1:-1]
	async with aiohttp.ClientSession() as session:
	try:
	async with session.get(url) as r:
	data = await r.read()
	except aiohttp.InvalidURL:
	return await ctx.send(_("That URL is invalid."))
	except aiohttp.ClientError:
	return await ctx.send(_("Something went wrong while trying to get the image."))