(core) add minimal support for activation keys

Summary: For grist-ee, expect an activation key in environment variable `GRIST_ACTIVATION` or in a file pointed to by `GRIST_ACTIVATION_FILE`. In absence of key, start a 30-day trial, during which a banner is shown. Once trial expires, installation goes into document-read-only mode.

Test Plan: added a test

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3426

app/common/gristUrls.ts

@@ -518,6 +518,23 @@ export interface GristLoadConfig {
 
   // Google Tag Manager id. Currently only used to load tag manager for reporting new sign-ups.
   tagManagerId?: string;
+
+  activation?: ActivationState;
+}
+
+/**
+ * For a packaged version of Grist that requires activation, this
+ * summarizes the current state. Not applicable to grist-core.
+ */
+export interface ActivationState {
+  trial?: {               // Present when installation has not yet been activated.
+    days: number;         // Max number of days allowed prior to activation.
+    daysLeft: number;     // Number of days left until Grist will get cranky.
+  }
+  needKey?: boolean;      // Set when Grist is cranky and demanding activation.
+  key?: {                 // Set when Grist is activated.
+    daysLeft?: number;    // Number of days until Grist will need reactivation.
+  }
 }
 
 // Acceptable org subdomains are alphanumeric (hyphen also allowed) and of