Opt out of previewing on a per msg basis

Current behavior

A user is unable able opt-out of their media links being previewed when one is sent in a channel that previews are enabled on.

Desired behavior

When a user wraps a media link in <>, the bot will not attempt previews for it.

Implementation detail

When reporting is enabled, the message_previewable event ought to be sent. In this case, preview ought to be blocked after reporting checks are run, but before a reply is made.

When reporting is disabled, the message_previewable event doesn't need to be emitted, and returning at that point in the meta-dispatcher will block the preview effectively.

If #76: "Support multiple links eligible for preview in a single message" is completed before this task is worked, it ought to be taken into account such that a subset of the total number of links may be excluded from preview.