Metadata commands

[mike-kaimika]

This PR adds 4 "commands" to the metadata import: add_dataset, remove_dataset, add_tag, remove_tag

1. The commands work with or without the underscore, and with "delete" as an alternative to "remove"
2. Attempting to add a dataset that does not exist throws an error for that row
3. Removing a tag or dataset that is not already associated with that bin will ignore it silently
4. There are checks in place for non-super admins (team captains) to ensure the dataset they want to add or remove is one they have access to. If not, all other data will be applied but not the dataset specified
5. Added some helper text to the import screen with what columns are available
6. Added these new columns to the export of metadata information