Use details on demand definitions for glossary

[larsyencken]

What

Replace the current glossary backend with details on demand in the grapher db.

Why

The glossary visual design is well loved, but authors wanted a better experience before committing to using it more. Then after some time, details on demand emerged for Grapher charts, and supports a dedicated admin. But these two features as so related that we believe they can share the same data backend.

If the same definition could be used in text, in charts and in explorers, then it would be much more attractive for researchers to use across our site.

What's involved

TBD (@mlbrgl do you have a sense on how hard this would be?)