[SVCS-143] WB V1 API: add links for ?meta= call on folders

[TomBaxter]

Purpose:

https://openscience.atlassian.net/browse/SVCS-143 WB V1 API: add links for ?meta= call on folders

Changes:

update waterbutler/core/metadata.py move base_api_links out of BaseMetadata into utils for reuse update waterbutler/server/api/v1/provider/metadata.py to return links for folder meta calls update waterbutler/server/utils.py add base_api_links for reuse

Side effects:

None

TODO

Should I move update waterbutler/core/metadata.py def _entity_url into utils as well? Could be confusing as self is file/folder object in waterbutler/core/metadata.py but self is provider handler object in waterbutler/server/api/v1/provider/metadata.py

[#OSF-5822]