Adds ability to specify default tab display via dataset-level config file

[jsheunis]

This is a development in response to https://github.com/psychoinformatics-de/sfb1451-projects-catalog/issues/36

This introduces several changes, including:

• the content tab is displayed in position 0
• the subdatasets tab is displayed in position 1
• other standard / non-standard tabs follow in their original order
• if no default tab is specified via the dataset-level config, the content tab is shown
• if a different default tab is provided via dataset-level config, that tab will be shown
• the default tab will be shown on:
  • navigation to dataset via URL
  • navigation to dataset from within the catalog web app
  • navigation to superdataset via user pressing the home button
• a new config file is added for the demo superdataset, with subdatasets as the default tab

Demo at: https://jsheunis.github.io/datalad-catalog-staging

The main things to notice:

• the content tab is in the first position, followed by the subdatasets tab
• the catalog home page dataset displays the subdatasets tab by default (because of the added config file)
• all other dataset pages display the content tab by default