Add provisioning api for bridging guilds

This isn't exactly the same as the puppet bridge, basically it removes the
puppetID from the url and just works against the logged in user.

There is a known errata right now where some times all channels aren't left
when unbridging a guild. I figured it's more important to get some more testing
on this while I try to figure that out in the mean time.

Also when you call /guilds/:guildID/joinentire, it joins each channel serially.
I tried to make this concurrent but I maxed out the rate limit. We can do this
in the future, but again, rather get this into everyone's hands sooner rather
than later. I also did the same thing with unbridge and had to revert for the
same reasons.

Refs #8

bridge/discord.go

@@ -0,0 +1,16 @@
+package bridge
+
+import (
+	"github.com/bwmarrin/discordgo"
+)
+
+func channelIsBridgeable(channel *discordgo.Channel) bool {
+	switch channel.Type {
+	case discordgo.ChannelTypeGuildText:
+		fallthrough
+	case discordgo.ChannelTypeGuildNews:
+		return true
+	}
+
+	return false
+}