[Content] Add a toggle to the header image to not overlay the site colour

**Is your feature request related to a problem? Please describe.**
Some users might prefer to have no color overlayed over the image from the `Header with background image` block.

**Describe the solution you'd like**
A toggle (default on `true`) on the content block should allow control over whether the `mix-blend-multiply` overlay appears over some images, coloring them to the site's theme.

**Describe alternatives you've considered**
Additionally, the ability to have a color selector like the divider might benefit some users.

**Additional context**
Demo of what an image would look like with and without color: