(core) Showing links in text cells

Summary:
When there is a link in a text cell (and formula cells), it will be
rendered with a little clickable icon wrapped in the anchor tag
with a proper link. Only links that starts with https? will be
rendered as links.

Links are shown in a Text and Formula fields, inside a GridView,
CardView and in the Import preview dialog.

Test Plan: Browser tests

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: dsagal, alexmojaki

Differential Revision: https://phab.getgrist.com/D3070

app/client/ui2018/icons.ts

@@ -68,6 +68,15 @@ const iconDiv = styled('div', `
   background-color: var(--icon-color, black);
 `);
 
+export const cssIconBackground = styled(iconDiv, `
+  background-color: var(--icon-background, inherit);
+  -webkit-mask: none;
+  & .${iconDiv.className} {
+    transition: inherit;
+    display: block;
+  }
+`);
+
 export function icon(name: IconName, ...domArgs: DomElementArg[]): HTMLElement {
   return iconDiv(
     dom.style('-webkit-mask-image', `var(--icon-${name})`),