Make external lemma link configurable

[mbollmann]

Originally reported by: Marcel Bollmann (Bitbucket: mbollmann, GitHub: mbollmann)

---

When is the external lemma link shown, and could we generalize this somehow? Pretty sure the URL is still hardcoded somewhere in there.

---

• Bitbucket: https://bitbucket.org/mbollmann/cora/issue/45

[mbollmann]

The latest commit(s) makes the URL customizable in the database, though we don't have frontend support for configuring this just yet.

[mbollmann]

For now, I've committed a change that simply hides the icon for the link when no URLs are defined in the database. There is still no front-end for setting these URLs without directly meddling with the database, but at least users won't ever see anything about lemma links (like non-functional icons etc.) by default.

Therefore, until we add an interface for configuring the URLs, we can simply consider this an undocumented feature.