[FLINK-5485] [webfrontend] Mark compiled web frontend files as binary…

… when processed by git diff

Particularly beneficial now that javascript is minified, we can mark
compiled web frontend files as binary when processed by git diff.
  https://linux.die.net/man/5/gitattributes

This does not affect how files are displayed by github.

This closes apache#3122

.gitattributes

@@ -1,3 +1,3 @@
 *.bat text eol=crlf
-flink-runtime-web/web-dashboard/web/* linguist-vendored
+flink-runtime-web/web-dashboard/web/* linguist-vendored -diff