WebStyle: new /goto URL handler

• Introduces new /goto URL handler. This is a plugin-based framework for URL redirectors, allowing a per-user registration of URLs under /goto/ path that will dynamically resolve to a final URL based on pluggable algorithms.

• Implements three plugins:
  • goto_plugin_simple, to implement a simple URL shortner;
  • goto_plugin_latest_record, to redirect to the latest record in a given collection or query;
  • goto_plugin_cern_hr_documents as an example of complex plugin that is used at the CERN Document Server.

• Implements the gotoadmin CLI. (closes #1178)

• Merge note: amended tests and left two of them in FIXME state. Need to touch them later.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>