Use StringBuilder for GetString/GetStringWithArgs, as per upstream

Update dependent code as required

src/network/network_chat_gui.cpp

@@ -33,10 +33,6 @@
 
 #include "../safeguards.h"
 
-/** The draw buffer must be able to contain the chat message, client name and the "[All]" message,
- * some spaces and possible translations of [All] to other languages. */
-static_assert((int)DRAW_STRING_BUFFER >= (int)NETWORK_CHAT_LENGTH + NETWORK_NAME_LENGTH + 40);
-
 /** Spacing between chat lines. */
 static const uint NETWORK_CHAT_LINE_SPACING = 3;