Migrate away from CodeIgniter3, update to newer web standards

[tenzap]

Kalkun is based on the CodeIgniter 3 (CI3) framework. Its code is hosted at: https://github.com/bcit-ci/CodeIgniter

CI3 releases are becoming rare or coming a long time after PHP releases which can be a problem because of the deprecations introduced in each new version of PHP.

Any help is welcome to

• migrate away from CI3
• update Kalkun to newer Web standard (other framework (CodeIgniter 4?, other?) , responsive webdesign...)
• migrate away from xhtml doctype. The files are not served as application/xhtml+xml but mostly as text/html. So browsers don't parse it with their XML parser anyway. Moreover, it is not possible to inject HTML code into the document structure by the means of jQuery when the files are parsed by XML. See https://stackoverflow.com/a/35384581. So serving the files as application/xhtml+xml would break kalkun.

There is no specific tool or framework in mind. However The components used in Kalkun should have DFSG compatible licenses.

I think for that change we don't have to stick supporting PHP 5.6 as Kalkun 0.8 currently does. The minimal required version of PHP can be bumped.

Anyone having ideas and willing to develop it can comment on Discussion #515.