Removed placeholders

Until now, we gradually replaced placeholders (i.e., User Interface elements with static content) with dynamic values provided by the backend. Now that enough UI elements with dynamic values exist, we are able to remove the remaining placeholders without having a layout that feels unpopulated.

We removed:

• the add page button next to the breadcrumb
• the help button and groups for the left sidebar
• the information tab
• the like and comments icons and count at the top left

The page edition details below the title have been replaced by dynamics values from the backend.