Rework linking/canonical logic based on app/canonical tag

[aaroncox]

Canonical Links: Add canonical links to posts based on the app tag or json_metadata. For reference, the discussion in steemit/condenser#336 is the basis of these changes.

User Preference: Default link behavior. Let the user toggle between options that modify links to Steem blockchain content. Options including:

• Option A: Prevent any modification of links and use whatever the author created the links as.
• Option B: Change all links based to whatever platform it was created on (via the app tag). e.g steemit.com links added to a Busy.org post would be rewritten to Busy.org.
• Option C: Forcing all links to chainBB. Any other frontends domain in the URL will be overwritten allowing the user to always remain on chainBB.

The user preference will not modify the canonical URL generated by chainBB.