This is a sub-task of moving front facing files to controllers.

As a result of a discussion on IRC, I'm proposing to move report.php into a controller under:

/report/{mode}/{id}

/report/pm/1

/report/post/3

...

Edit: as a result of an other discussion on IRC, it will be

/post/{id}/report

/pm/{id}/report

Also moving report.php's database handling into services.