Allow popups and links within mermaid diagrams

This fixes  the broken links within sandboxed mermaid
diagrams. This is due to iframe sandbox feature

Changelog: fixed

app/assets/javascripts/behaviors/markdown/render_sandboxed_mermaid.js

@@ -88,7 +88,7 @@ function renderMermaidEl(el, source) {
   const iframeEl = document.createElement('iframe');
   setAttributes(iframeEl, {
     src: getSandboxFrameSrc(),
-    sandbox: 'allow-scripts',
+    sandbox: 'allow-scripts allow-popups',
     frameBorder: 0,
     scrolling: 'no',
     width: '100%',

spec/features/markdown/sandboxed_mermaid_spec.rb

@@ -26,7 +26,7 @@ RSpec.describe 'Sandboxed Mermaid rendering', :js do
 
     wait_for_requests
 
-    expected = %(<iframe src="/-/sandbox/mermaid" sandbox="allow-scripts" frameborder="0" scrolling="no")
+    expected = %(<iframe src="/-/sandbox/mermaid" sandbox="allow-scripts allow-popups" frameborder="0" scrolling="no")
     expect(page.html).to include(expected)
   end
 end