Add ability of using `[p]modset deleterepeats` per channel

[PredaaA]

Feature request

Select the type of feature you are requesting:

• [ ] Cog
• [x] Command
• [ ] API functionality

Describe your requested feature

Currently, behavior of [p]modset deleterepeats (mod cog) is guild wide. Though it would be useful to be able to set that only per channel too. I don't think it'd needs a new command, just add it as an argument in the existing command.

[Jackenmen]

This shouldn't be too hard to implement as part of the existing delete repeats check and [p]modset deleterepeats command and I don't see much against it really if someone has a use case for it.

One note here: to be able to differ between "inherit from the guild" and "disable delete repeats check in this channel", we need to use separate values to set both of these cases in case of the channel.

[YannPollet]

Could I work on this please? I'd suggest to have a map "delete_repeats_channels" that maps the id of the channel and the number of deletes. That means that there will be the default value and you can customize for each channel after (-1 will still ignore repeated messages).