Followup from #16562 prepare for #16567
* Rename ctx.Form() to ctx.FormString()
* Reimplement FormX func to need less code and cpu cycles
* Move code into own file
* refactor routers directory
* move func used for web and api to common
* make corsHandler a function to prohibit side efects
* rm unused func
Co-authored-by: 6543 <6543@obermui.de>
