Added a link from each opportunity to its latest Find a Tender notice

Opportunities from Find a Tender have a link icon before their title.
Clicking it fetches the opportunity with its releases (the list leaves
them out) and opens its newest release's notice,
www.find-tender.service.gov.uk/notice/<release_id>, in a new tab. The
tab is opened on the click and pointed at the notice when the request
returns, so it isn't blocked as a popup; if the request fails, or there
are no releases, it's closed and a toast says why. Opportunities from
other sources have no link for now, but keep its space so the titles
line up.

The icon is lucide-solid's, added for it at 1.52.0.

web/src/lib/opportunities.ts

@@ -1,4 +1,4 @@
-import type { Opportunity } from '@/types/Opportunities';
+import type { Opportunity, OpportunityRelease } from '@/types/Opportunities';
 
 /**
  * The opportunities with changed merged in: each replaces the opportunity
@@ -12,3 +12,27 @@ export function mergeOpportunities(
   for (const opportunity of changed) byUuid.set(opportunity.uuid, opportunity);
   return [...byUuid.values()];
 }
+
+/** The source the backend gives opportunities fetched from Find a Tender. */
+const FIND_A_TENDER_SOURCE = 'FindATender';
+
+/** Whether the opportunity was fetched from Find a Tender, so its releases are notices there. */
+export function isFromFindATender(opportunity: Opportunity): boolean {
+  return opportunity.source === FIND_A_TENDER_SOURCE;
+}
+
+/** The opportunity's newest release, or undefined if it has none. */
+export function latestRelease(opportunity: Opportunity): OpportunityRelease | undefined {
+  let latest: OpportunityRelease | undefined;
+  for (const release of opportunity.releases ?? []) {
+    if (!latest || Date.parse(release.release_date) > Date.parse(latest.release_date)) {
+      latest = release;
+    }
+  }
+  return latest;
+}
+
+/** The page of a Find a Tender notice; a Find a Tender release's ID is its notice's. */
+export function findATenderNoticeUrl(releaseId: string): string {
+  return `https://www.find-tender.service.gov.uk/notice/${encodeURIComponent(releaseId)}`;
+}