Add symbolic link storage

* Adds a symbolic link storage for developers. To enable it, put this
  to your config: COLLECT_STORAGE = u"flask.ext.collect.storage.link"
  then run your `collect` script to create all the links from the
  static folders to the intance folder.

* NOTE only files are symlinked, not folders!

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Co-authored-by: Kamil Neczaj <kamil.neczaj@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>

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cc original authors @greut and @kneczaj