markdown_link_slow(): use _force_unicode
markdown_link_slow(): escape brackets
OAuth2 API: add scope/endpoints for using creddits

This adds a new OAuth2 scope, "creddits", and two endpoints that use it:

/api/v1/gold/give/{username} (with optional months amount)
/api/v1/gold/gild/{fullname}
Replace gold_creddit_escrow with creddits_lock