BibCirculation: set colour of some buttons

* Explicitly sets the background for some buttons to avoid the
  possibility of having white text on white background.
CKEditor: updated to version 3.6.6
BibSched: bugfix for high-priority monotasks

* Fixes a bug where a high-priority monotask was not able to be executed
  until all lower priority tasks had actually completed.
WebSearch: 410 HTTP code for deleted records

* When the URL for a record that was actually deleted is accessed
  returns a 410 HTTP error code together with the human oriented message.

* Fixes typos in HTTP error codes definition.
WebLinkBack: fix truncated FSF address in docstring
dateutils: new get_time_estimator function

* Introduce a new useful function called get_time_estimator
  which allows to easily compute time estimation for long
  tasks.
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>
Merge branch 'maint-1.1'

* maint-1.1:
  WebSearch: 410 HTTP code for deleted records
  BibSched: bugfix for high-priority monotasks
  CKEditor: updated to version 3.6.6
Merge branch 'master' into next

* master:
  WebStyle: new /goto URL handler
  dateutils: new get_time_estimator function
  WebLinkBack: fix truncated FSF address in docstring
  WebSearch: 410 HTTP code for deleted records
  BibSched: bugfix for high-priority monotasks
  CKEditor: updated to version 3.6.6
  BibCirculation: set colour of some buttons