jupyterhub 1.0

Update to Spark 2.4.3

Update RTD config file to v2

Community Stacks documentation ihaskell-notebook

Add ihaskell-notebook to the list of Community Stacks.

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.