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".
BUG Fixing installer not checking display_errors correctly.

Fixes issue #2479. Installer sets display_errors on, but it checks
the changed value and not the original one set in php.ini.
Merge pull request #2328 from chillu/pulls/versioned-archivemode-validation

Validate 'archiveDate' user data in Versioned
Merge pull request #2288 from chillu/pulls/browser-spellcheck

API Disable discontinued Google Spellcheck in TinyMCE (#2213)
Merge pull request #2487 from halkyon/installer_error_check_30

BUG Fixing installer not checking display_errors correctly.
BUG Fixed cross-platform issues with test cases and file utilities
Updating unit and integration testing documentation.

Updating introduction summary, and fixing broken links.
Updated translations
Merge branch 'refs/heads/3.1.0' into 3.1
Merge pull request #2488 from tractorcow/pulls/3.0-platform-compatibility

BUG 3.0 Fixed cross-platform issues with test cases and file utility
Corrected translation references in templates

Were using old entity naming scheme (*.ss) instead
of underscored version which doesn't create a new YML
namespace. This means all existing translations should indeed be used.
Was already fixed in master, but not in 3.1
More mappings for i18n::tinymce_lang
Behat test for language switching
Updated translations
Added 3.0.8 changelog
Added 3.1.1 changelog
Merge pull request #2500 from halkyon/doc_updates

Updating unit and integration testing documentation.
Updated translations
Added 3.1.1 changelog