Use .condarc to install extra packages

[humitos]

conda has an option to install packages by default in all the environments created: https://docs.conda.io/projects/conda/en/latest/user-guide/configuration/use-condarc.html#always-add-packages-by-default-create-default-packages

It seems that we can use this option instead of what we are currently doing:

1. running an extra conda install command, or
2. appending our core requirements to the user's environment.yml (feature flag CONDA_APPEND_CORE_REQUIREMENTS)

This has some benefits:

• run only one command to create the whole environment
• reduce build time
• use a standard way to install default packages
• remove our hacky code that does exactly the same
• no need to activate the feature flag per-project

This config was added in 2.0.1, we can start using it without too much problem with the current docker images in production

Reference: readthedocs/readthedocs-docker-images#113