adjust HttpException path
use Assert class to simplify registration code
use User::getID() to shorten code
remove obsolete line of code
add a Registration class for easier handling
simplify registration code with new class
rework mail template

Instead of a POST parameter, define the template in the server config.
Change the variable style to be consistent with suspensions after mail
change. Use arrays with str_replace() instead of a loop. Add a new con-
fig for the mail subject line.
delete obsolete function
add missing require() for Assert.php
fix require() path
split registration API code in 3 files (one for each step)

Although this produces slightly more code, it is way easier to maintain.
Throw 404 if registration session cannot be found