Improve and make more usable the idea of having inter-page links with [[page]]

[docelic]

The wiki already supports internal links via [[page]].

This model should be made more prominent and useful within the whole user interface.

(E.g. below the ToC there could be 3 lists -- one as a collection of links to internal pages, one as a collection of external links, and one as a collection of referenced files from the internal media library)

[docelic]

Since we use MDE's internal previewer which (I believe) does not recognize [[page]] links, we should override its previewRender option to call our custom renderer.

The custom renderer would call the default one, and then just additionally handle the [[page]] links.

[docelic]

In addition to converting internal links to HTML links, the custom renderer should also support stuff from here which isn't yet supported (and/or SimpleMDE/InscrybMDE should be patched to support it):

https://guides.github.com/features/mastering-markdown/