wikibase: Alert the user when they do not provide an edit summary

[t8210103]

Closes #6306

Changes proposed in this pull request:

• Unfortunately, i couldn't configure the request for a working "required" tag inside the HTML code.
• Directly implementing the "required" tag in the appropriate HTML "input" tag did not work, since the form isn't directly submitted from the HTML code, but from a JavaScript code that doesn't recognize the "required" tag.
• Although, as i noticed in different parts off the project, problems as such are dealt with the pop-up window that follows:
  
  -(The previous message pops-up when the user clicks "next" without choosing a file in the "create project" section).
  -What this PR changes, is instead of focusing to the box next to "summary", it pops-up the following message:
  
  -(This validation message was already implemented inside the code but never reached).

[wetneb]

Thank you for having a look!

Reusing the validation logic from the backend is not a bad idea, but I am reluctant to have it include a Java exception class name because it's an internal implementation detail that isn't very readable and therefore shouldn't be presented to users.

Also, opening a modal dialog strikes me as rather "obstructive" since it requires the user to first close the modal, then manually focus the summary field, before they can start describing their changes. Don't you like pointing the user to the field directly? It feels like a widespread practice to me.

[t8210103]

Absolutely, I understand your concerns. I will close this pull request for now, and if I manage to address these points, I'll open a new one.