templatetag: add the default metadata value to the tag rendering context

This is better than trying to use the default metadata for empty values on
the current metadata, since they may be empty for a reason.
models: added custom manager to have a shortcut for the default metadata

Use that instead everywhere we need it and change the condition for the default
metadta to have view_name and content_type as null
models: title should be required
admin: fix errors and required display