webmap is now in maps.txt instead of a config entry (#90821)

## About The Pull Request

Currently the button to open a map in webmap is tied to a config entry,
which would then take the name of the map being played and get the link
to that, however for cases like new/tested/admin uploaded maps where
there is no webmap available, this would still have a link to a url that
doesn't exist.
Instead, we'll make the webmap link be in ``maps.txt``, same as
feedbacklink.

## Why It's Good For The Game

Explained in the about section, this prevents webmaps appearing for maps
that aren't supposed to have a webmap link available.

## Changelog

Nothing player-facing, webmaps aren't currently used (afaik) currently
cause they're broken for tg codebases.

config/maps.txt

@@ -11,7 +11,8 @@ Format:
 	voteweight [number] (How much to count each player vote as, defaults to 1, setting to 0.5 counts each vote as half a vote, 2 as double, etc, Setting to 0 disables the map but allows players to still pick it)
 	disabled (disables the map)
 	votable (is this map votable)
-	feedbackurl (link in-game shown to players to leave feedback for the map)
+	feedbacklink (link in-game shown to players to leave feedback for the map)
+	webmap_url (link to the a webmap to see the map in the user's browser)
 endmap
 
 # Production-level maps.
@@ -42,6 +43,7 @@ map metastation
 	#voteweight 0.5
 	votable
 	#feedbacklink https://www.youtube.com/watch?v=XG8b7WhANNA
+	#webmap_url https://webmap.affectedarc07.co.uk/maps/tgstation/metastation
 endmap
 
 map tramstation