add a button in administration to clean filecache

File cache is used for test step sequences diagram, pdf generation, but sometimes the generated file is bad, could be due to wrong response from application and this bad file is cached and served at each requests.
Clients then receve the bad file even after application restart.

The fix is to clean the file cache with: `rm -r /opt/gazelle/fileCache/*`

It will help a lot to add a button in the admin area to clean this cache.