for any additional blog post, add test that checks if all necessary metadata was added to the document

[TimDaub]

Scope

• add metadata to blogs #13 added metadata to blogs. e.g. "author" and "keywords"
• However, when someone now submits a new post, it isn't mandatory for them to add any of this metadata
• But for the RPI backend, it may become mandatory to have this metadata for each post available

Deliverables

• Write a test that checks for each blog post if all necessary metadata "title", "author" and "keywords" is defined and at least "truthy"

Time estimate

• 2h