Add preliminary report system

The report system is pretty low-tech. However the scaffolding is there
for a lot of new stuff. What we currently have:

* Users can create reports
* Staff can view reports
* Admins can create report templates

There's a post drop-down menu available on all posts now, too. This is
where "report post" menu item lives and other things like that can be
added too.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/models.py

@@ -212,7 +212,7 @@ class ReportReason(models.Model):
     # Urgency. If true, this post is probably reported as illegal content.
     urgent = models.BooleanField(default=False)
     # This is a board-specific report reason
-    board = models.ForeignKey("Board", on_delete=models.CASCADE, null=True)
+    board = models.ForeignKey("Board", on_delete=models.CASCADE, null=True, blank=True)
 
     def __str__(self):
         return self.reason