Add disk usage statistics

- add model CustomerPackageDiskUsage for hosting package disk usage
  statistics
- add REST API endpoint for submittings statistics for disk, mysql and
  pgsql usage
- add disk usage information to hosting package detail view
- add separate hosting package disk usage statistic view

gnuviechadmin/hostingpackages/models.py

@@ -269,6 +269,31 @@ class CustomerHostingPackage(HostingPackageBase):
             return DISK_SPACE_FACTORS[unit][min_unit] * diskspace
         return DISK_SPACE_FACTORS[min_unit][unit] * diskspace
 
+    disk_space = property(get_disk_space)
+
+    def get_used_disk_space_sum(self, unit=None):
+        """
+        Get the used disk space of this hosting package from submitted disk space statistics.
+
+        :param unit: value from :py:data:`DISK_SPACE_UNITS` or :py:const:`None`
+        :return: disk space in unit or bytes (if parameter unit is :py:const:`None`)
+        :rtype: int
+
+        """
+        sum = 0
+        for usage in self.customerpackagediskusage_set.all():
+            sum += usage.size_in_bytes
+        if unit is None:
+            return sum
+        return DISK_SPACE_FACTORS[0][unit] * sum
+
+    used_disk_space_sum = property(get_used_disk_space_sum)
+
+    def get_space_level(self):
+        return self.used_disk_space_sum / self.disk_space * 100.0
+
+    space_level = property(get_space_level)
+
     def get_package_space(self, unit=None):
         """
         Get the total disk space reserved for this package without looking at
@@ -474,3 +499,34 @@ class CustomerMailboxOption(MailboxOptionBase, CustomerHostingPackageOption):
         help_text=_("The mailbox option template that this mailbox option is based on"),
         on_delete=models.CASCADE,
     )
+
+
+class CustomerPackageDiskUsage(TimeStampedModel):
+    """
+    This class represents disk usage statistics for a customer hosting package.
+    """
+
+    package = models.ForeignKey(
+        CustomerHostingPackage,
+        verbose_name=_("hosting package"),
+        help_text=_("The hosting package"),
+        on_delete=models.CASCADE,
+    )
+    source = models.CharField(
+        verbose_name=_("data source"),
+        choices=(("disk", _("disk")), ("mysql", _("mysql")), ("pgsql", _("pgsql"))),
+    )
+    item = models.CharField(verbose_name=_("data item"))
+    used_kb = models.PositiveBigIntegerField(
+        verbose_name=_("space used in KiB"), default=0
+    )
+
+    class Meta:
+        unique_together = ("package", "source", "item")
+
+    def __str__(self):
+        return "%s %s = %d KiB" % (self.source, self.item, self.used_kb)
+
+    @property
+    def size_in_bytes(self):
+        return self.used_kb * 1024