Makes client upload limit a config, bumps admin file size upload limit to 5mb (from 2.5mb) (#77708)

## About The Pull Request
Makes client upload limit a config. Increases max file size admins can
upload to 5mb, up from 2.5mb.
## Why It's Good For The Game
Makes it easier to do custom maps for events. Some of our in-game maps
are already larger than 2.5mb (delta is almost 3mb).
## Changelog
🆑
admin: admins can now upload files to the server up to 5mb (used to be
2.5mb)
config: file upload restriction is now config
/🆑

code/controllers/configuration/entries/general.dm

@@ -701,3 +701,11 @@
 
 /datum/config_entry/flag/config_errors_runtime
 	default = FALSE
+
+/datum/config_entry/number/upload_limit
+	default = 524288
+	min_val = 0
+
+/datum/config_entry/number/upload_limit_admin
+	default = 5242880
+	min_val = 0