load data UI: issues related to upload metadata

[TiangeZhu]

1. might need to define priority while reading json files in zip: right now the top is metadata extracted from scores. as for metadata from json files, we don't care if corpus.json is first or doc_id.json is first read in the zip for now, that means: If corpus.json and doc_id.json both exists in a zip, and the composer info and corpus.json is read first, composers of all the files in the zip are updated according to composer info in corpus.json, if the scores don't contain this information. Later when doc_id.json is read, it will detect that the composer info is already not null or unknown composer, so it won't update. This can cause potential problem seen in the case described below:
2. potential case where there's error, for (1): if not all opus in a zip is composed by the same person, but when we upload a corpus file, it contains the corpus.json which is read before other doc_id.json, it can cause error in composer metadata info of opuses in this zip!!! should we define the priority here? Make sure doc_id.json are read before corpus.json. However, it can make the code a bit more complicated.
3. corpus.json is not allowed for upload metadata module for data protection. should this be changed?
4. infos other than title and composer are ignored for now from json files. Can change when needed.