MDL-33550 Correctly process situation when file reference source is missing

- do not die with fatal error if source file in moodle internal repository is missing;
- moved code duplication for moodle repositories into class repository (functions send_file, get_reference_details, get_file_by_reference, get_file_reference);
- update file status after repository::sync_external_file so we know that it is missing (or not missing anymore). Do not run this function more than once for file within one request;
- display readable name for Private Files and Server files with the new format;
- display broken icon in filemanager if we know that source is missing, display information (for admin) where it was located before: see repository::get_reference_details() and extending classes;
- removed unnecessary queries in stored_file::sync_external_file();
- syncronize files before displaying it's size in mod_resource, do not query  directly

lib/filelib.php

@@ -596,6 +596,9 @@ function file_get_drafarea_files($draftitemid, $filepath = '/') {
             $item->datemodified = $file->get_timemodified();
             $item->datecreated = $file->get_timecreated();
             $item->isref = $file->is_external_file();
+            if ($item->isref && $file->get_status() == 666) {
+                $item->originalmissing = true;
+            }
             // find the file this draft file was created from and count all references in local
             // system pointing to that file
             $source = unserialize($file->get_source());
@@ -2310,7 +2313,7 @@ function send_stored_file($stored_file, $lifetime=86400 , $filter=0, $forcedownl
     }
 
     // handle external resource
-    if ($stored_file->is_external_file()) {
+    if ($stored_file && $stored_file->is_external_file()) {
         $stored_file->send_file($lifetime, $filter, $forcedownload, $options);
         die;
     }