New feature: implement search for public libraries

[kelockhart]

Public libraries are currently only visible if someone directly shares the link. This issue would add a search feature to make public libraries discoverable.

Things to consider:

• should current libraries with the public permission be discoverable, or only libraries with a new discoverable permission?
• what fields should be indexed? (e.g. title, description, owner, etc.) Should the search be implemented just with postgres?
• how to encourage users to make metadata descriptive for other users? A pop-up encouraging descriptive title/description?