Backend: Read page content from an external source

[jlehtine]

Add a backend option for reading page content from an external source.

Rationale: Support use cases where the page related user manual is not included on the web page itself but is available elsewhere.

Environment variable: VW_PAGE_CONTENT_URL

If the environment variable is defined then fetch data from the specified URL and use it as page content, instead of the page content provided by frontend.

If both VW_PAGE_CONTENT_URL and VW_PAGE_CONTENT are defined then try to read page content from URL but if that fails then use the statically configured page content.