Fix duplicate folder creations during import (#4702)

During import you are able to select an existing folder, or with
Bitwarden exports it can contain existing folders already. In either
case it didn't matter, we always created new folders.

Bitwarden uses the same UUID of the selected or existing folders if they
are already there.

This PR fixes this by using the same behaviour.

Fixes #4700

src/api/core/folders.rs

@@ -41,6 +41,7 @@ async fn get_folder(uuid: &str, headers: Headers, mut conn: DbConn) -> JsonResul
 #[serde(rename_all = "camelCase")]
 pub struct FolderData {
     pub name: String,
+    pub id: Option<String>,
 }
 
 #[post("/folders", data = "<data>")]