Fixes #4791
Previously, our rich text conversion functions handled the case where a document link specified an ID which is not found in the database. However, they failed with a KeyError when the id attribute was missing completely; links of this second type would occur whenever a link of the first type was re-saved from the Draftail editor. The fix is two-fold:
1) Catch the "missing ID attribute" case - in this case, the resulting link will be missing both the href and id attributes
2) Update the handling of the "ID present but document not found" case so that the id attribute survives the round-trip to the editor and back. The final link as rendered on the front-end will still be an attribute-less <a> element, but the id will be retained in the database (and in the versions rendered within rich text editors) which may be useful for troubleshooting.
Added important note about using url() on older versions of django, but switched the examples to re_path as to comply with Django docs for 2.0 and later.
Currently queries executed in the hooks don't run in the transaction
with the page deletion query and it's harder to write hook without
copying the whole view if you want to keep queries running in the hooks
integral with page deletion.
According to a google search I just did, it seems a lot of people have forgotten to add ``Disallow: /admin`` in their robots.txt (or forgot to add robots.txt) at all.
Adding this meta tag into the head of all admin pages should prevent any admin pages being indexed even if this was missed.
The behaviour of `has_usable_password` has changed in Django 2.1, such that `None` is no longer considered a 'non-usable' password: https://docs.djangoproject.com/en/2.1/ref/contrib/auth/#django.contrib.auth.models.User.has_usable_password
As a consequence of the fix applied in Django https://code.djangoproject.com/ticket/28718 , Wagtail users created without a password will now be able to complete the password reset process to gain access to Wagtail. Sites that do not want this behaviour (e.g. because those users should be using an LDAP login instead) should disable password changes via WAGTAIL_PASSWORD_MANAGEMENT_ENABLED and WAGTAIL_PASSWORD_RESET_ENABLED.
* Update integrating_into_django.rst
flaw in url, was still referencing depreciated method, fixed.
* Update integrating_into_django.rst
Add important notice that versions of Django earlier than 2.0 require url() instead of re_path()
