Escape custom Open Graph tags

HedgeDoc allows to specify custom Open Graph tags using the
`opengraph` key in the YAML metadata of a note.

These are rendered into the HTML delivered to clients using `ejs` and
its `<%-` tag. This outputs the variable unescaped into the template
and therefore allows to inject arbitrary strings,
including `<script>` tags.

This commit changes the template to use ejs's `<%=` tag instead,
which automatically escapes the variables content,
thereby mitigating the XSS vector.

See also GHSA-gjg7-4j2h-94fq

Co-authored-by: Christoph (Sheogorath) Kern <sheogorath@shivering-isles.com>
Signed-off-by: David Mehren <git@herrmehren.de>

public/views/hedgedoc/head.ejs

@@ -7,7 +7,7 @@
 <%- include('../includes/favicon') %>
 <% for (var og in opengraph) { %>
 <% if (opengraph.hasOwnProperty(og) && opengraph[og].trim() !== '') { %>
-<meta property="og:<%- og %>" content="<%- opengraph[og] %>">
+<meta property="og:<%= og %>" content="<%= opengraph[og] %>">
 <% }} if (!opengraph.hasOwnProperty('image')) { %>
 <meta property="og:image" content="<%- serverURL %>/icons/android-chrome-512x512.png">
 <meta property="og:image:alt" content="HedgeDoc logo">