rapidjson: remove outdated in-tree, use submodule

Includes ~2 years worth of fixes including security and build.

Note: this pulls directly from master, not a release branch/tag. See
https://github.com/Tencent/rapidjson/issues/1265

(cherry picked from commit 8a6c081df7)

.gitmodules

@@ -6,3 +6,6 @@
 	path = external/miniupnp
 	url = https://github.com/monero-project/miniupnp
 	branch = monero
+[submodule "external/rapidjson"]
+	path = external/rapidjson
+	url = https://github.com/Tencent/rapidjson