router: fix /accounts collision again, gracefully fail for others

all routes are registered on the root of the host, and the "reverse
proxy subfolder". When using Reverse proxy subfolder "/accounts", and
the default URL Path for the Admin page of "/" (a.k.a. ""), a route
collision occurs that for some reason wasn't dealt with before. This is
avoided by checking before registering the second route, and a panic
recovery handler has been added telling the user off for using bad
subpaths if any others occur.

logmessages/logmessages.go

@@ -212,6 +212,7 @@ const (
 	FailedInitProxy     = "Failed to initialize proxy @ \"%s\": %v\nStartup will pause for a bit to grab your attention."
 	NoURLSuffix         = `Warning: Given "jfa_url"/"External jfa-go URL" value does not include "url_base" value!`
 	BadURLBase          = `Warning: Given reverse proxy subfolder "%s" may conflict with the applications subpaths.`
+	RouteCollision      = `Route Collision! Given reverse proxy subfolder "%s" or "URL Paths" settings likely conflict with the applications subpaths. Culprit: %v`
 	NoExternalHost      = `No "External jfa-go URL" provided, set one in Settings > General.`
 	LoginWontSave       = ` Your login won't save until you do.`
 	SubpathBlockMessage = `URLs: Root subfolder = "%s", Admin = "%s", My Account = "%s", Invite forms = "%s"`