Merge pull request #336 from john-hill/PLFM-1780

Plfm 1780
Merge pull request #337 from jay-hodgson/develop

SWC-248: Generate appropriate previews for all leaf entities
Merge pull request #340 from jay-hodgson/release-1.21

cherry-pick SWC-248 for release branch
PLFM-1787
Add EZID configuration
Merge pull request #344 from brucehoff/release-1.21

PLFM-1787
New doi package. EzidMetadata
Add EZID client
Add testing invalid doi
PLFM-1787: updates to integration test users
Merge pull request #345 from brucehoff/release-1.21

PLFM-1787: updates to integration test users
Add retry with exponential backoff
Add mocked test for retry
Merge pull request #347 from eric-wu/develop

Add a retryable HTTP client over EZID's REST APIs
PLFM-1787
Merge branch 'PLFM-1787' into develop
Merge pull request #348 from brucehoff/develop

PLFM-1787
Add DOI data objects
Add skeleton code for DOI controller, service
Manager chunk file complete