Separate extended metadata fields from functionality

[floradanna]

Currently, every metadata checklist used for extended metadata in SEEK is seeded in the database.

For ENA upload tool extended metadata #1589 , it is best to separate the functionalities (e.g., extended metadata for 1st assay only) from the actual checklist.

The checklist should be similar to ISA Template:

• a separate file
• stored in ELIXIR BE GitHub repo
• that can be uploaded in a specific instance of SEEK

[floradanna]

relate to #1121