Custom Message Templates Revert to Default If Missing Slugs

[barrycarey]

This was intended behavior to prevent a user from inputting a bag slug and causing the message to blow up.

However, since different submission types can have additional message slugs this can cause missing slugs depending on submission type. As an example, if a link submission is checked and the custom message template contains {closest_percent_match} (only available on Image types) the template builder fails and reverse to the default message.

The cleanest solution is to allow Subreddit's to set a message template per submission type so we can enforce what slugs are used. This has the added benefit of allowing better targeted messages for things like galleries.