Fix bug preventing movement of channels when there are gaps in position

The original code was too aggressive in ensuring that the channel
positions are correct and within bounds. Unfortunately, for Discord
this number is merely a guess and there can be gaps that cause the
position number to be greater than the number of channels currently
in that sorting bucket.

One such example of this is when a channel is deleted and created. When
this happens, the positions are not updated (presumably because Discord
considers this to be too expensive). Sadly this means that if a user
were to create a channel, delete a channel, and then create another one
then there will be gaps in the position sequence.

More concretely, consider the following example:

Channels with position [0, 1, 2, 3, 4]. A new channel is created so it
inherits the maximum position giving us [0, 1, 2, 3, 4, 5]. We delete
a channel that's smaller than the maximum so our list now looks like
[0, 1, 2, 4, 5]. Next time we create a channel we will further
increment the gap, giving us [0, 1, 2, 4, 5, 6]. If a user would want
to move the channel with position 4 to be after position 6 (so in this
case that value would 7) then the library would erroneously raise an
error because it assumed too much about the data integrity.

Luckily, the library upon actually doing the request fixes the channel
positions so everything goes back to normal like it was intended. That
is to say, [0, 1, 2, 3, 4, 5].

discord/abc.py

@@ -202,9 +202,6 @@ class GuildChannel:
         bucket = self._sorting_bucket
         channels = [c for c in self.guild.channels if c._sorting_bucket == bucket]
 
-        if position >= len(channels):
-            raise InvalidArgument('Channel position cannot be greater than {}'.format(len(channels) - 1))
-
         channels.sort(key=lambda c: c.position)
 
         try:
@@ -214,8 +211,9 @@ class GuildChannel:
             # not there somehow lol
             return
         else:
+            index = next((i for i, c in enumerate(channels) if c.position >= position), -1)
             # add ourselves at our designated position
-            channels.insert(position, self)
+            channels.insert(index, self)
 
         payload = []
         for index, c in enumerate(channels):
@@ -259,7 +257,7 @@ class GuildChannel:
                 options['permission_overwrites'] = [c._asdict() for c in category._overwrites]
         else:
             await self._move(position, parent_id=parent_id, lock_permissions=lock_permissions, reason=reason)
-        
+
         overwrites = options.get('overwrites', None)
         if overwrites:
             perms = []