[ADD] legal: update contracts for new maintenance

- Update Enterprise & Partnership agreement (EN, FR)
- Update legal page index: hide outdated agreements, replace
  with a link to an archive of the old PDFs.
- Add icons for missing files, to make index table layout more consistent
- Add `text-justify` on HTML versions of contracts, for consistency with
  the PDF versions.
- Fix compatibility of PDF generation with recent sphinx versions
  (\sphinxmaketitle instead of \maketitle)
- Re-add NL PDF building, still needs to be updated like all others

_static/legal.css

@@ -0,0 +1,18 @@
+
+section#terms-and-conditions table td {
+    /* make sure icons and links aren't wrapped */
+    white-space: nowrap;
+}
+
+section#terms-and-conditions table th {
+    text-align: center;
+}
+
+section#terms-and-conditions table img.img-responsive {
+    margin: 0px;
+}
+
+section#terms-and-conditions table td:nth-child(2),
+section#terms-and-conditions table th:nth-child(2) {
+    background-color: #f3f3f3;
+}