Allow custom logo integration on client site

This patch allows a user to upload an image to be displayed in the header of the client interface. The logo is stored in the database as a file with ft='L'. This also changes the `AttachmentFile::deleteOrphans` function to only consider aTtachment file types where `ft=='T'`

It introduces a `logo.php` and `scp/image.php` script to serve file content from the database, and `AttachmentFile::display` now supports client-side caching with the `Last-Modified`, `ETag`, `If-Modified-Since`, and `If-None-Match` headers. `AttachmentFile::display` will return HTTP/304 if the client-side cache should be used and doesn't download the data from the database.