Do not overwrite sphinx context variables feature

[davidfischer]

This adds a feature flag (default off) that changes how html_context variables are clobbered in conf.py. By default, we clobber whatever the user may have in html_context if it matches variable names set by Read the Docs. This changes the setting to respect what a user may have and only overwrite if the user didn't set anything.

[agjohnson]

Nice! Looks good to me 👍

I think if we did a sample of projects, this will be a good test that helps us answer how backwards compatible we need to be when implementing a versioned context_data.

[davidfischer]

I think if we did a sample of projects, this will be a good test that helps us answer how backwards compatible we need to be when implementing a versioned context_data.

Agreed

[ericholscher]

I think if we did a sample of projects, this will be a good test that helps us answer how backwards compatible we need to be when implementing a versioned context_data.

It's going to be almost impossible to tell if these builds fail or have unexpected behavior because of the change though. If we logged or collected metrics on the number of times we hit a user-specified key that would probably be the most valuable.

[humitos]

If we logged or collected metrics on the number of times we hit a user-specified key that would probably be the most valuable

I like it. This metric could help us to understand the context better than a failed build. Do we have a table for saving this kind of things already?

[humitos]

Good and simple enough!

I want to mention that I would like to do the same for the readthedocs.v1 context by default (the new design) and remove the warning from the bottom of this page: https://docs.readthedocs.io/en/latest/design/theme-context.html#customizing-the-context and allow users to override only one key and not the whole readthedocs.v1 object.

Of course, this is something to be done in a completely different PR.

[davidism]

There's a typo, SPINX -> SPHINX.

[davidfischer]

The typo is fixed.