[github issues] Add page section to template URL

[pvalena]

By appending the values for subdirectory in _config.yml .

If we would want to be more fine-grained, we should be able to do that also via _config.yml by just appending the values for the specific subdirectory.

Originally posted by @pvalena in #140 (comment)

[pvalena]

This could be also possibly (but not so simply) reused for jekyll headers instead of specifying it explicitly on each page.