Fix urlPath support, let CodiMD be served from a subpath correctly

Webpack now uses relative paths for resources linked from by static
snippets. A templated <base> tag has been introduced in headers
so app.js can set the base URL at runtime.

Signed-off-by: Enrico Guiraud <enrico.guiraud@cern.ch>

public/views/codimd/head.ejs

@@ -12,6 +12,7 @@
 <meta property="og:image:alt" content="CodiMD logo">
 <meta property="og:image:type" content="image/png">
 <% } %>
+<base href="<%- serverURL %>/">
 <title><%= title %></title>
 <link rel="icon" type="image/png" href="<%- serverURL %>/favicon.png">
 <link rel="apple-touch-icon" href="<%- serverURL %>/apple-touch-icon.png">