Support multiple URL schemes for previewing articles in Communities

[dextermilo]

Currently we support previewing articles in a single context using a single scheme which is not well suited for multiple communities and/or multiple community types.

Domains can be configured on a community by community basis and they can also be shared. When they are shared, the site 'pathprefix' setting is used to separate them.

For example:

https://powerofus.force.com
https://powerofus.force.com/someothercommunity 

Article URLs for these communities might be:

https://powerofus.force.com/{articleId}
https://powerofus.force.com/someothercommunity/{articleId}

For a sandbox this might look like:

https://sandboxname-powerofus.cs81.force.com/{articleId}
https://sandboxname-powerofus.cs81.force.com/someothercommunity/{articleId}

Completely custom domains can also be used:

https://somecustomdomain.com/{articleId}

That said, there is always a 'subdomain' configured for each site/community that works on *.force.com.

The customer may also want to maintain the option of previewing the article in their standard Salesforce instances:

{instanceUrl}/{articleId}

Some customers might be interested in providing their own preview URL if they deliver content to a separate application or platform. This seems like a less critical issue to solve.