all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/syncthing/notify/AUTHORS

@@ -0,0 +1,10 @@
+# List of individuals who contributed to the Notify package.
+#
+# The up-to-date list of the authors one may obtain with:
+#
+#   ~ $ git shortlog -es | cut -f2 | rev | uniq -f1 | rev
+#
+
+Pawel Blaszczyk <blaszczykpb@gmail.com>
+Pawel Knap      <pawelknap88@gmail.com>
+Rafal Jeczalik  <rjeczalik@gmail.com>