Add more links on the bottom left of the home page

[fflorent]

Describe the problem to be solved

In #925, we introduced the ability to add Terms of Service link of the instance on the home page.

In fact, we would like to add more (and probably specific) resources alongside the ToS (like accessibility declarations or legal notice)

Describe the solution you would like

We would like to be autonomous to configure these buttons.

I can think of a environment variable which would contain a simple JSON object, like:

GRIST_HOME_PAGE_RESOURCES='{"Accessibility declaration": "https://...", "legal notice": "https://..."}'

The key would be the label and the value the link to the page for each button, which would be added complying to the order set in the JSON object.