Allow adding multiple same type of content blocks on page

Currently the home page allows defining the content using the customizable or domain specific content blocks. Some of these blocks are by nature designed so that they should only appear on the page once but there is no reason why this restriction exists for some of the blocks. Generally the type of blocks that could appear multiple times are those that allow the user to define the content for them. This includes currently: - The hero image - HTML block The users might want to add multiple hero images or multiple HTML blocks (or multiple other types of customizable blocks) on the content pages. Therefore, it should be possible to do so from the UI.