Fossil SCM

The purpose of the attach-v2 branch is to modernize the file attachment capabilities, in particular for the forum:

1. Attach multiple files at a time.
2. Attach via drag/drop.
3. Attach screenshots and text pasted from the clipboard.
4. Provide thumbnails for being-added image attachments (as distinct from the list of current attachments, where we cannot do so without downloading the whole attachment image and letting the browser scale it down).

With those in place, we will be able to rework the forum post editor to be able to attach files during the initial edit, instead of having to save the post before attaching files. That will come in a later step/branch, after this one is usable.