Catch 'jav\tascript:alert("XSS")' as a bad URL

The URL checker from html5lib was used as inspiration. As html5lib is
based on the same ideas that browsers (should) use for parsing and
checking, this solution should be much more robust.
Removed 'wagtailsearch.indexed'
Updated deprecation warnings
Fixed bad import
Tabbing from password field goes to Submit
Fix typo in wagtailforms.forms.FormBuilder.formfields
Merge pull request #713 from nimasmi/fix_wagtailforms_formfields

Fix typo in wagtailforms.forms.FormBuilder.formfields
Search fields can be overridden again. Fixes #709
Added system check for ForeignKey cascade

Pages can be linked to other Pages, Images, etc through ForeignKeys. If
the object being referenced by a page gets deleted, the page itself will
be deleted as well. Also, the page will not be cleanly removed from the
tree (leaving orphans, out of date num_child, etc). It is very common to
accidentally forget to set the on_delete action to models.SET_NULL.

This pull request checks all ForeignKey fields on pages and makes sure
that they are not set to cascade. If they are set to cascade, the
developer will be warned of their mistake.

This only works for Django 1.7 users as it uses the system checks
framework.
Merge pull request #700 from kaedroho/remove-features-08

Removed 'wagtailsearch.indexed' module and updated deprecation warnings
Use latest_revision_created_at in sitemap gen

Much faster than looking it up manually for every page in the site
Fixed login form to reference USERNAME_FIELD

Was hardcoded to say 'username' when USERNAME_FIELD may be 'email' etc.
Fixed JSON Oembed error on Python 3

=> TypeError: the JSON object must be str, not 'bytes'
Fixed failing oembed tests
Updated LoginForm to dynamically set placeholder text

Placeholder text is now determined from the verbose name of AUTH_USER_MODEL's USERNAME_FIELD
Added 0.8 release notes to index
Auto register signals only works on Django 1.7
Wagtail 0.8 is in development
Added failing test
created_at of PageRevisions is now copied on page copy