Ckanext-dc-serve

- fix: download buttons are broken
- tests: increase coverage
- ref: optimize serving data in serve.py

- feat: move template code for resource download buttons here from the
dcor_theme extension
- feat: only show condensed resource download button if the resource
has been condensed (1)

- enh: introduce resource file locks on /tmp to avoid condensing
the same resource in different processes

- fix: add redis connection

- fix: prevent enqueueing of same background job twice

- maintenance release