Introduce twig macros for commonly used form elements

Quite a few form elements like select or input are currently created inside PHP files. Part of this is done due to the need to autogenerate these fields. However, twig does come with form macros that can be used instead and allow part of this auto generation (read that as: the HTML part) while compiling the template files.

This will allow adjusting HTML and styling elements better without the need to directly modify PHP files. It should also enable styling parts of phpBB's forms which was so far not possible.

Due to the size of required changes, this ticket is about first introducing the concept and first macros. Follow up tickets and PRs will build upon these and continue the work of moving HTML code from PHP files.