Bug 36845: Exclude meta tag from the translations

This bug originaly wants to get rid of "noindex" coming from this meta
tag:
  <meta name="robots" content="noindex">

But actually we have other strings from the meta tags that should not be
translated.

Test plan:
0. Do not apply this patch
1. cd misc/translator/po && gulp po:update --lang es-ES (or any other
   lang)
2. git commit -a -m"wip"
3. Apply this patch
4. Repeat 1 and git diff to show the diff
Notice that strings that should not be translated are removed from the
po files (actually commented)

Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit fbb123b12e)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

misc/translator/xgettext.pl

@@ -135,9 +135,10 @@ sub text_extract {
             for my $a ('alt', 'content', 'title', 'value', 'label', 'placeholder', 'aria-label') {
                 if ($attr->{$a}) {
                     next if $a eq 'label' && $tag ne 'optgroup';
-                    next if $a eq 'content' && $tag ne 'meta';
+                    next if $a eq 'content';
                     next if $a eq 'value' && ($tag ne 'input'
                         || (ref $attr->{'type'} && $attr->{'type'}->[1] =~ /^(?:hidden|radio|checkbox)$/)); # FIXME
+                    next if $tag eq 'meta';
                     my($key, $val, $val_orig, $order) = @{$attr->{$a}}; #FIXME
                     $val = TmplTokenizer::trim($val);
                     # for selected attributes replace '[%..%]' with '%s' globally