[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

Author: greghogan

.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