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Changed `conda clean` flags to `--all -f -y`

Resolved conflicts between local branch and updates to the upstream
master.

More translation fixes

* msgstr fields should be empty
* File extension should be .po

Do not trigger another docs/local/en update and push
to master when localized text changes

Need to check file changes in TRAVIS_COMMIT_RANGE

Draft pages about translation

Setup travis en template update

[Doc] Change Spark Python Notebook entry point sentence

make clean gettext
sphinx-intl update -p _build/gettext -l en

Build docs, images when appropriate

This commit adds the additional `-f` force command to all uses of `conda
clean --all` through the repo. Size should be smaller, but still testing
if anything breaks. See issue #861.

The last commit was only for the base-notebook's Dockerfile. For this,
all the files in the repo were grepped through and changed.

Following the discussion in issue #861, this changes the `conda clean`
flags.

tx pull -all manually

Update to Spark 2.4.2

Update to JupyterLab 0.35.5

Until we figure out what I did wrong with the GitHub integration setup:
https://community.transifex.com/t/github-pr-goes-to-wrong-folder/366

This contains a fix for chrome download file bug: https://github.com/jupyterlab/jupyterlab/pull/6232#issuecomment-486618455

Try RTD+Transifex workflow