FEATURE: Improve backup stats on admin dashboard

* Dashboard doesn't timeout anymore when Amazon S3 is used for backups
* Storage stats are now a proper report with the same caching rules
* Changing the backup_location, s3_backup_bucket or creating and deleting backups removes the report from the cache
* It shows the number of backups and the backup location
* It shows the used space for the correct backup location instead of always showing used space on local storage
* It shows the date of the last backup as relative date

lib/backup_restore/backup_store.rb

@@ -18,7 +18,7 @@ module BackupRestore
 
     # @return [Array<BackupFile>]
     def files
-      unsorted_files.sort_by { |file| -file.last_modified.to_i }
+      @files ||= unsorted_files.sort_by { |file| -file.last_modified.to_i }
     end
 
     # @return [BackupFile]
@@ -26,6 +26,11 @@ module BackupRestore
       files.first
     end
 
+    def reset_cache
+      @files = nil
+      Report.clear_cache(:storage_stats)
+    end
+
     def delete_old
       return unless cleanup_allowed?
       return if (backup_files = files).size <= SiteSetting.maximum_backups
@@ -33,6 +38,8 @@ module BackupRestore
       backup_files[SiteSetting.maximum_backups..-1].each do |file|
         delete_file(file.filename)
       end
+
+      reset_cache
     end
 
     def remote?
@@ -60,6 +67,15 @@ module BackupRestore
       fail NotImplementedError
     end
 
+    def stats
+      {
+        used_bytes: used_bytes,
+        free_bytes: free_bytes,
+        count: files.size,
+        last_backup_taken_at: latest_file&.last_modified
+      }
+    end
+
     private
 
     # @return [Array<BackupFile>]
@@ -70,5 +86,13 @@ module BackupRestore
     def cleanup_allowed?
       true
     end
+
+    def used_bytes
+      files.sum { |file| file.size }
+    end
+
+    def free_bytes
+      fail NotImplementedError
+    end
   end
 end