refactor /events/add POST handling
refactor /events/add POST handling

* make it more modular so that you can easily have different handlers
to validate the data
* after a failed add, go back to mainpage, cause i don't have time to
do it properly right now

jjardel/anthracite,square-root/anthracite,jjardel/anthracite,nickman/anthracite,jjardel/anthracite,nickman/anthracite,jjardel/anthracite,nickman/anthracite,square-root/anthracite,Dieterbe/anthracite,nickman/anthracite,square-root/anthracite,Dieterbe/anthracite,Dieterbe/anthracite,Dieterbe/anthracite,square-root/anthracite