URL-encode executorId query parameter in web UI thread dump pages

This fixes an issue where the executorId `<driver>` wasn't properly url-encoded in the web UI, which led to issues when proxying the driver thread dump page.

/cc @henrydavidge
Chrome can interpret `http://example.com:8088/proxy/application_1415344371838_0006/executors/threadDump/?executorId=<driver>` to `http://example.com:8088/proxy/application_1415344371838_0006/executors/threadDump/?executorId=%3Cdriver%3E` automatically. I don't know if it's same in other browser. Therefore I think it's safer and I will add it to #3183