FIX Avoid infinite loops on ?isDev=1 and Deprecation class

If any of the functionality triggered by Director::isDev()
was causing deprecation errors, the system would go into
an infinite loop. Since the only way to cause this is the DB
checking functionality, we disable that for Deprecation.
Side effect of this change: You can't show deprecation notices
on a live site by forcing the session into dev mode.
API Disable discontinued Google Spellcheck in TinyMCE

Replaced by browser-based spellchecking if available (Chrome, Firefox),
with instructions on how to use PSpell as an alternative.
Validate 'archiveDate' user data in Versioned

Not a security issue as such, since the user input is sanitized
before being used in Versioned->augmentSQL(). But it shouldn't
reach the session state either, since that's commonly assumed
to be sanitized data, and it leaves unnecessary room for error.

strtotime() has fairly loose validation rules around dates,
but its a good "first line of defence".
"edit" form expansion in AssetUploadField

Form wasn't expanding because of fixed heights. Backported fix from 3.1.
Update 2-extending-a-basic-site.md

Fixed a few wording issues and added some clarity to links eg: Tutorial One (Building a basic site)
Count, not Length
Merge pull request #2400 from jbridson/patch-9

Update 2-extending-a-basic-site.md
FIX Privilege escalation through Group hierarchy setting (SS-2013-003)

See http://www.silverstripe.org/ss-2013-003-privilege-escalation-through-group-hierarchy-setting/
FIX Privilege escalation through Group and Member CSV upload (SS-2013-004)

See http://www.silverstripe.org/ss-2013-004-privilege-escalation-through-group-and-member-csv-upload/
FIX Privilege escalation through APPLY_ROLES assignment (SS-2013-005)

See http://www.silverstripe.org/ss-2013-005-privilege-escalation-through-apply-roles-assignment/
Linking to older security issue in change log

Mainly for consistency with the newer format
FIX Disallow permissions assign for APPLY_ROLES (SS-2013-005)

See http://www.silverstripe.org/ss-2013-005-privilege-escalation-through-apply-roles-assignment/
Safety note on DataObject::validation_enabled
Merge branch 'pulls/security-issues-august-3.0' into 3.0
Updated translations
Added 3.0.6-rc2 changelog
Tagged 3.0.6-rc2
Update 3.0.6.md

Add reference to information disclosure in Versioned.php (SS-2013-006)
Merge pull request #2413 from ss23/patch-1

Update 3.0.6.md
BUG Fixing tabindex added to CreditCardField when tabindex is NULL

The tabindex increment *should* only be done if there is a tabindex
that has been set on a CreditCardField already, otherwise it breaks
the tab ordering.