[FLINK-4304] [runtime-web] Jar names that contain whitespace cause problems in web client

[twalthr]

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I tested the webfrontend with a Jar named ' "!@#$%^&*()_öäß.jar. Uploading, submitting, showing plan, and deleteting should work with this PR.

[mxm]

+1 LGTM

[StephanEwen]

+1

[twalthr]

Thanks for reviewing. Merging...