WIP apply category layout to activities

[carlobeltrame]

Fixes #7140

To be explored

• What to do after the dialog has been used and the progress bar has completed? Close the dialog? Display a confirmation message?
• How to calculate the "last updated" timestamp? The updated_at column of what entities need to be taken into account here?
• Could we detect when updated_at and created at of the whole content node tree are close enough together and in that case display "Created at ..., no changes since" instead?
• What is an appropriate way to textually summarize the presence of contents of an activity? How to generate it? For now I just have a list of used content types, but this at best describes the layout, and does not give any insight on contents.
• Could we detect "equality" in layout and content by transforming the content node tree into an appropriate data structure with omitted URIs, and then comparing them with deep equals? We then could disable the checkboxes of activities which already match the category.
• If we already generate such a URI-less form of a content node tree, could we create a diff between the category's tree and the activity's tree, and use that for generating an advanced summary text for the user?
• Could we use that same information to effectively merge the changes to the category layout into the activities? Or is the original tree at the time of creating the activity required for that calculation? If so, is there an easy way for us to keep track of this original state (should be per activity) or of all the change steps?
• Is it better to do such content node tree comparisons in the backend or is it okay to do it in the frontend?