synapse guest module: change license

Set license to multi-license of AGPL + Element Commercial, and add
associated license files to the repository root.

This does not add a notice of "This file includes modifications" to any
files because they were only moved and relicensed, but otherwise
unmodified.

The exception is run_in_venv.sh, which is a rewrite of upstream's
run_in_venv.js and thus lacks the original copyright entirely.

modules/restricted-guests/synapse/setup.cfg

@@ -4,9 +4,10 @@ description = A synapse module to restrict the actions of guests
 long_description = file: README.md
 long_description_content_type = text/markdown
 version = 1.0.0
-
-classifiers =
-  License :: OSI Approved :: Apache Software License
+license = AGPL-3.0-only OR LicenseRef-Element-Commercial
+license_files =
+    ../../../LICENSE-AGPL-3.0
+    ../../../LICENSE-COMMERCIAL
 
 
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