Finalize ticket relations (closes issue 638)

- IssueUpdate.php: use dynamically set field validators for dynamically
  created fields; let relation_type0 and relation_issue0 exist at any time;
  check the validity of a user selection and combine the various input fields
  if possible; do the database updates for links; change the "change" format
  for labels to a more precise structure and no longer trust on a leading
  dash for removed labels
- IssueCreate.php: change the validator calls and field names
- Issue.php (getGroupedRelatedIssues): make it possible to return only a
  flat list of integers for easier processing
- 17AddIssueRelations.php: migrate the previous serialized "changes" format
  for issue comments to the new, more structured format (up and down)
- js-autocomplete.html: add support for multiple input fields
- view.html: output relation changes and wrap the related issues stanzas into
  paragraphs
- NEWS.mdtext: note the addition and the need for a specific version of Pluf

src/IDF/templates/idf/issues/create.html

@@ -63,12 +63,12 @@
 </td>
 </tr>
 <tr>
-<th>{$form.f.relation_type.labelTag}:</th>
+<th>{$form.f.relation_type0.labelTag}:</th>
 <td>
-{if $form.f.relation_type.errors}{$form.f.relation_type.fieldErrors}{/if}
-{if $form.f.relation_issue.errors}{$form.f.relation_issue.fieldErrors}{/if}
-{$form.f.relation_type|unsafe}
-{$form.f.relation_issue|unsafe}
+{if $form.f.relation_type0.errors}{$form.f.relation_type0.fieldErrors}{/if}
+{if $form.f.relation_issue0.errors}{$form.f.relation_issue0.fieldErrors}{/if}
+{$form.f.relation_type0|unsafe}
+{$form.f.relation_issue0|unsafe}
 </td>
 </tr>
 <tr>