make Help a little bit context sensitive

[williamstein]

When you open a file and click the Help button in Cocalc, a modal dialog is displayed with some bullet points and a box where you can ask for help.

We might as well have some extra bullet points that take into account the file being edited. For example, if you're editing a .lean file, add this link:

• Lean Zulip chat https://leanprover.zulipchat.com/ (login required, real names preferred, be nice) -- Here you find a community of mathematicians and computer scientists who are working out how to communicate with each other, and are isolating specific problems in the way of getting fashionable mathematics into Lean, and then solving them. Somebody there will be able to answer your question.

I copied the above sentence from [here](https://xenaproject.wordpress.com/2020/02/09/where-is-the-fashionable-mathematics/. We could have similar pointers for Sage, each Jupyter kernel, etc.