Standardize how defaults are assigned

There's a problem that meta tag defaults are assigned in different ways - in some cases they are assigned at the time of content creation, e.g. the Dublin Core tags, while in other cases they are not added until the output is being generated. This is honestly a little dumb and it should work one way for all use cases:

• Default values are only inserted at the time of output,
• Default values are not inserted into edit forms,
• However, edit forms should display what the default value is, taking into consideration how content types and vocabularies can override this.

It may also be worth adding additional caching to the default values, given how often they are loaded.