Messages older than 2 weeks can't be purged with `/purge`

When purging messages older than 2 weeks, the following exception would be thrown:
```
System.ArgumentOutOfRangeException: Messages must be younger than two weeks old. (Parameter 'MessageIds')
at Discord.Preconditions.YoungerThanTwoWeeks(UInt64[] collection, String name)
at Discord.API.DiscordRestApiClient.DeleteMessagesAsync(UInt64 channelId, DeleteMessagesParams args, RequestOptions options)
at Discord.Rest.ChannelHelper.DeleteMessagesAsync(ITextChannel channel, BaseDiscordClient client, IEnumerable`1 messageIds, RequestOptions options)
at XeniaBot.Core.Modules.ModerationModule.PurgeMessages(Int32 count, IChannel channel) in /src/XeniaBot.Core/Modules/ModerationModule.cs:line 335
```
https://github.com/ktwrd/XeniaBot/blob/a9ba5d1264c1c9fb25c5dcf6825c0d1be827d91a/XeniaBot.Core/Modules/ModerationModule.cs#L335

This should be fixed so mods can use the `/purge` command. But this needs to be done once BanSync PostgreSQL data migration has been deployed & ran.

https://docs.discord.com/developers/resources/message#bulk-delete-messages

Might need to implement a queue for this operation in a channel, since only 5 messages per second can be deleted.

Start in XeniaBot.Core/Modules/ModerationModule.cs at line 335 and read Discord's bulk-delete message documentation. Confirm that the BanSync PostgreSQL migration has been deployed and run, then determine how /purge should handle messages older than two weeks and the five-per-second deletion limit. Done means moderators can purge those messages without the shown exception.

Written by the indexing model from the issue text.