Fix RelPermalink() and Urls in menus vs canonifyUrls

Fix RelPermalink() and Urls in menus vs canonifyUrls

canonifyUrls=true, RelPermalink and baseUrl with sub-path did not work.

This fixes that by adding a check for canonifyUrl=trues=true in RelPermalink().

So given

- baseUrl "http://somehost.com/sub/"
- the path "some-path/file.html"

For canonifyUrls=false RelPermalink() returns "/sub/some-path/file.html"
For canonifyUrls=true RelPermalink() returns "/some-path/file.html"

In the last case, the Url will be made absolute and clickable in a later step.

This commit also makes the menu urls defined in site config releative. To make them work with canonifying of urls, the context root is prepended if canonifying is turned off.

Fixes #519
Fixes #711