[Cleanup] Add [p]cleanup between (#2617)

* Add `[p]cleanup between`

At the moment, only deletes the messages in between, does not delete the messages specified when running the command.

* Create converter for message IDs; remove f-strings; use converters in a few extra commands

* Sacrifice to the style gods

redbot/cogs/cleanup/converters.py

@@ -0,0 +1,12 @@
+from redbot.core.commands import Converter, BadArgument
+from redbot.core.i18n import Translator
+
+_ = Translator("Cleanup", __file__)
+
+
+class RawMessageIds(Converter):
+    async def convert(self, ctx, argument):
+        if argument.isnumeric() and len(argument) >= 17:
+            return int(argument)
+
+        raise BadArgument(_("{} doesn't look like a valid message ID.").format(argument))