media assets: move more file-handling logic into MediaAsset.

Move more of the file-handling logic from UploadService and
StorageManager into MediaAsset. This is part of refactoring posts and
uploads to allow multiple images per post.

app/logical/storage_manager/local.rb

@@ -3,7 +3,7 @@ class StorageManager::Local < StorageManager
   DEFAULT_PERMISSIONS = 0o644
 
   def store(io, dest_path)
-    temp_path = dest_path + "-" + SecureRandom.uuid + ".tmp"
+    temp_path = full_path(dest_path) + "-" + SecureRandom.uuid + ".tmp"
 
     FileUtils.mkdir_p(File.dirname(temp_path))
     io.rewind
@@ -11,17 +11,17 @@ class StorageManager::Local < StorageManager
     raise Error, "store failed: #{bytes_copied}/#{io.size} bytes copied" if bytes_copied != io.size
 
     FileUtils.chmod(DEFAULT_PERMISSIONS, temp_path)
-    File.rename(temp_path, dest_path)
+    File.rename(temp_path, full_path(dest_path))
   rescue StandardError => e
     FileUtils.rm_f(temp_path)
     raise Error, e
   end
 
   def delete(path)
-    FileUtils.rm_f(path)
+    FileUtils.rm_f(full_path(path))
   end
 
   def open(path)
-    File.open(path, "r", binmode: true)
+    File.open(full_path(path), "r", binmode: true)
   end
 end

app/logical/storage_manager/rclone.rb

@@ -35,6 +35,6 @@ class StorageManager::Rclone < StorageManager
   end
 
   def key(path)
-    ":#{remote}:#{bucket}#{path}"
+    ":#{remote}:#{bucket}#{full_path(path)}"
   end
 end

app/logical/storage_manager/sftp.rb

@@ -19,6 +19,7 @@ class StorageManager::SFTP < StorageManager
   end
 
   def store(file, dest_path)
+    dest_path = full_path(dest_path)
     temp_upload_path = dest_path + "-" + SecureRandom.uuid + ".tmp"
     dest_backup_path = dest_path + "-" + SecureRandom.uuid + ".bak"
 
@@ -42,7 +43,7 @@ class StorageManager::SFTP < StorageManager
 
   def delete(dest_path)
     each_host do |_host, sftp|
-      force { sftp.remove!(dest_path) }
+      force { sftp.remove!(full_path(dest_path)) }
     end
   end
 
@@ -50,7 +51,7 @@ class StorageManager::SFTP < StorageManager
     file = Tempfile.new(binmode: true)
 
     Net::SFTP.start(hosts.first, nil, ssh_options) do |sftp|
-      sftp.download!(dest_path, file.path)
+      sftp.download!(full_path(dest_path), file.path)
     end
 
     file