Clean up content model - Githubissues

Fixes #23

[x] Merge page and t-default into t-page
[x] Rename seo to d-seo
[x] Rename name fields to internal_name
[x] Add help text for internal_name fields
[x] Make internal_name fields required
[x] Remove content type section
[x] Limit the entries in m-columns (each column max. 1)
[x] Bump cssg packages to latest versions (especially contentful-migrations with the new field grouping feat.)
[x] Remove t-default Storybook example (open issue: page templates cannot be created in Storybook)
[x] Disable lazy load option of c-image and m-media for editors
[x] Update t-page migration and use field grouping (Compose only) to separate page and seo settings
[x] Disable theming option of t-page and all modules for editors
[x] Align naming of headlines with ui/ux (e.g. headline instead of title)
[x] Use HUGO_ENV instead of HUGO_ENVIRONMENT
[x] Remove the ability to change the logo via Contentful
[x] Disable theme option of t-page
[x] Disable hero content in m-stage
[x] Rename m-editorial to m-hero

jungvonmatt / wekit