Added support for custom error pages.
Added custom 404 error page.
Added more error handlers.
Merge remote-tracking branch 'gentlecat/errors'

* gentlecat/errors:
  Added more error handlers.
  Added support for custom error pages. Added custom 404 error page.
Added basic page with links to item.
Rewrite and attempt to make more concise the 'data' and 'data-lifecycle' documentation.
Remove obsolete weakness (now corrected) from mapping.rst
Document the database structure using sphinx autodoc and a bunch of docstrings.
Use headers less sparingly, for more useful linkable sections.
Merge branch 'doc-improvements'

* doc-improvements:
  Use headers less sparingly, for more useful linkable sections.
  Document the database structure using sphinx autodoc and a bunch of docstrings.
  Remove obsolete weakness (now corrected) from mapping.rst
  Rewrite and attempt to make more concise the 'data' and 'data-lifecycle' documentation.
Merge branch 'master' into links
Added map_dict property for Item.
Extracting value from map of linked item.
Improved content display on links page.
Replaced type column with actual names on links page.
Swapped columns Path and Value on links page.
Added link to page with links to item.
Added API endpoint for linked items.
Added basic tests for new views.
Created new macro render_title to remove duplicate code.
Removed more duplicate code.