Add a new view "forum attachments"

[frankystone]

I have recently searched an older image postet in the forum. But searching is cumbersome because one need to know at least the topic or part of the text of a post and probably the approximate date of the post.

There should be an additional page which shows all attachments of the forum in a list/table, newest first. The new page should contain:

1. a Filter to restrict the view to show only the types mentioned here, defaulting to images
2. beside the image/filename a link to the post where the attachment was postet and a link to the corresponding topic
3. for forum admins an additional button to delete the attachment

This new "Forum Attachments" view can be linked inside the dropdown "Forums" in the main menu. Not sure if this view should be available only for registered users or to all visitors.

[bunnybot]

Mirrored on Codeberg as #CB350.