Make Include for cover image & put icon in middle

**Short description**: Create an include for cover images, to be used for events & blog posts. Take plain image & put icon (default) or wordmark (if set on include) in the middle on top.

**Location**:

**Why have this**: Currently (since #220) the default cover image doesn't include an icon or wordmark. It was nice to have it. Also, it will make it easier to implement [random cover images](https://mariordev.com/articles/random-background-image/) if not set on post/page.

**More info**: