Add mentionable property to ModifyGuildRoleParams

Allows roles to be configured to be mentionable, fixes #424

per the following API error:
{
  "bitrate": [
    "Int value should be greater than 8000."
  ]
}

this was a big one

Adds Timestamps to embeds, and removes Image and Thumbnail-Builders

Close the WebSocket when disconnecting

This prevents a 400 when sending a malformed color to Discord.

**This is not a breaking change**.

This change begins to implement #379, where IUserMessage.ModifyAsync will now accept a `Discord.ModifyMessageParams` func over a `Discord.API.Rest.ModifyMessageParams` func.

This resolves #224