Support color string being when converting to hex code

Int he latest version of discord.py Embed colours could be sent to site as hex strings, if set using a discord.Color, rather tha an int.

Closes #715 Closes SITE-25

Author: ChrisLovering

pydis_site/apps/staff/templatetags/deletedmessage_filters.py

@@ -1,18 +1,22 @@
 from datetime import datetime
+from typing import Union
 
 from django import template
 
 register = template.Library()
 
 
 @register.filter
-def hex_colour(color: int) -> str:
+def hex_colour(colour: Union[str, int]) -> str:
     """
-    Converts an integer representation of a colour to the RGB hex value.
+    Converts the given representation of a colour to its RGB hex string.
 
     As we are using a Discord dark theme analogue, black colours are returned as white instead.
     """
-    colour = f"#{color:0>6X}"
+    if isinstance(colour, str):
+        colour = colour if colour.startswith("#") else f"#{colour}"
+    else:
+        colour = f"#{colour:0>6X}"
     return colour if colour != "#000000" else "#FFFFFF"