theme: restructure code

[McShelby]

The theme uses a proprietary mechanism to allow for adjusting parts of the layout for different output partials. This has a lot of drawbacks:

• users, familiar with Hugo need to learn a mechanism only used in the theme
• the way it's implemented complicates code (to a degree even Joe Mooring on the official Hugo forum made an comment about it ;-) )
• it restricts further development due to its complexity

The resulting capabilites of this mechanism can be implemented with all Hugo features, making it still complex but more common for advanced users and may profit from better caching (we'll see on that).

For that, the following changes will be implemented:

• remove internal output-partial mechanism by reverting back to standard partial calls
• simpify the archetype support by not clattering it in multiple files -> changing frontmatter archetype to type (which is a built-in Hugo frontmatter option)
• leverage baseof templates
• base it on Hugos .Render function
• renaming/moving templates meant for internal usage