(core) Add basic activation page to grist-ee

Summary:
Adds an activation page to grist-ee that currently shows activation status.

Follow-up diffs will introduce additional functionality, such as the ability to
enter activation keys directly from the activation page.

Test Plan: No grist-ee tests (yet).

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal, paulfitz

Differential Revision: https://phab.getgrist.com/D3582

app/client/ui/inputs.ts

@@ -0,0 +1,42 @@
+import {colors, vars} from 'app/client/ui2018/cssVars';
+import {dom, DomElementArg, Observable, styled} from 'grainjs';
+
+export const cssInput = styled('input', `
+  font-size: ${vars.mediumFontSize};
+  height: 48px;
+  line-height: 20px;
+  width: 100%;
+  padding: 14px;
+  border: 1px solid #D9D9D9;
+  border-radius: 4px;
+  outline: none;
+  display: block;
+
+  &[type=number] {
+    -moz-appearance: textfield;
+  }
+  &[type=number]::-webkit-inner-spin-button,
+  &[type=number]::-webkit-outer-spin-button {
+    -webkit-appearance: none;
+    margin: 0;
+  }
+
+  &-invalid {
+    border: 1px solid ${colors.error};
+  }
+
+  &-valid {
+    border: 1px solid ${colors.lightGreen};
+  }
+`);
+
+/**
+ * Builds a text input that updates `obs` as you type.
+ */
+export function textInput(obs: Observable<string>, ...args: DomElementArg[]): HTMLInputElement {
+  return cssInput(
+    dom.prop('value', obs),
+    dom.on('input', (_e, elem) => obs.set(elem.value)),
+    ...args,
+  );
+}