a8n: Add CampaignTypes description of diff in Markdown & return description in NPM credentials campaign

Created by: mrnugget

This implements the second and last part of the "Enhanced Workflow" described in https://github.com/sourcegraph/sourcegraph/issues/6852#issuecomment-560329073 and thus fixes #6852 (closed) (leaving aside the stretch goals, see #7010 (closed)).

What this PR does:

• Add a Description field to CampaignJobs that gets optionally populated by CampaignTypes to describe the diff the generateDiff method returned for a specific repository.
• Change the credentials CampaignType to return a description that includes a list of tokens that were removed
• Add the CampaignJob.Description to the Body of a pull request on GitHub and Bitbucket Server
• Downgrade from GFM when creating a PR on BBS by turning - [ ] taskitems into normal list items