Filter pages based on catalog parent-id and tags

[JavierCladellas]

• Closes Allow arbitrary nesting of modules  #79

Added a filter to helpers/get-page-cards.js that:

• Checks if the catalog tag is present in a page.
• Gets the identifiers of the parents pages by looking at the parent-catalogs attribute.
• Gets the actual identifier of the parent page, located as tags in the actual parent page attributes
• If the parent page contains any of the tags present in the parent-catalogs, it will be nested inside that module.

[ Example usage]
The work still uses the toolboxes page layout, but searches for the catalog tag to not interfere with existing work.

The base of the catalog system should have the following attributes, assuming I want to set the id of the page to base-id:

:page-layout: toolboxes
:page-tags: catalog, base-id

Then for any direct child of the base-id page :

:page-layout: toolboxes
:page-tags: catalog, toolbox, my-page-1-id
:parent-catalogs: base-id

If I want to create a card that appears both on the base-id and the my-page-1-id pages I can do:

:page-layout: default
:page-tags: catalog, toolbox, my-leaf-page
:parent-catalogs: base-id, my-page-1-id

= My Leaf page content... 

[NOTES]

• the page layout of all catalog pages that have children should be toolboxes
• All pages inside the catalog must have the catalog tag and the toolbox tag (for cards to appear)
• A page can have multiple parents.
• The page-layout of the leaf pages is not important

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[JavierCladellas]

Done in #81 without fork.