add convenience function for empty check without 0

This allows to quickly check if a variable is empty but keeps strings
and numbers evaluating to zero.

Fixed undefined error in html_denied()

Sorting by the sort number is too confusing since those are assigned
independently by different authors

Translation update (ru)

update Creative Commons licenses to 4.0. fixes #1286

https://wiki.creativecommons.org/wiki/License_versions
https://wiki.creativecommons.org/wiki/Frequently_Asked_Questions#Why_should_I_use_the_latest_version_of_the_Creative_Commons_licenses.3F

Translation update (sk)

Fix the flow of the navigation buttons in user manager

Fixed several undefined errors.

* Fixed identified issue with button spans by replacing missing closing span tag as identified by selfthinker, removed in ae614416.

* Changed the flow of the navigation button HTML so the navigation buttons actually flow to the right.

Translation update (bg)

Translation update (zh-tw)

Translation update (it)

Translation update (it)

Translation update (it)

Translation update (it)

Translation update (it)

Translation update (it)

Translation update (it)

Translation update (it)