Merge branch 'tmp-3.1.2' into 3.1
Merge pull request #2596 from camspiers/sstemplateparser-extendabilty

Allow users to extend the SSTemplateParser by defining open & closed blocks
FIX: scaffolded search field null selection by default

Enum search fields should default to having an empty string set
NEW: Pre-populate and disable foreign key field on has many lists.

This is a common use case, and by default a form field is added which
has no effect. While this coupling is undesirable, it makes the default
behaviour much more sensible.

See #2662, #2651, #2637 for more information.
NEW: GridField can sort on relation columns

Slightly improved logic

Add support for relations more than one 'level' apart

Add unit tests

Fixing PostgreSQL support

Throw exception if attempting to sort on a has_many/many_many relation
Add X-Backurl header to CMS ajax requests
Fixing coding conventions in Behat files
Merge pull request #2657 from kinglozzer/cms-xhr-referer

NEW: Add X-Backurl header to CMS AJAX requests
Fix installer isIIS() to include IIS versions greater than 7.
Merge pull request #2672 from tiaitsch85/tiaitsch85-patch-1
API: Implement SS_Map::push() to append values.

FIX: SS_Map::count() not taking into account additional firstItems
FIX text/html type for Debug::friendlyError() in live mode

Avoid unparsed HTML responses in case of httpError(4xx) or
httpError(5xx) in live mode. The SS_HTTPResponse_Exception constructor
automatically set "text/plain" a content type, which conflicts
with the later choice of Debug::friendlyError() to return HTML.

This is a hotfix, needs more work to properly separate
logic flow from presentation concerns (e.g. through request processors).
Merge pull request #2671 from wilr/ssmapfixes

API: Implement SS_Map::push() to append values.
Merge pull request #2654 from icecaster/patch-3

Fixed RSS example
Merge pull request #2674 from chillu/pulls/friendlyerror-contenttype

FIX text/html type for Debug::friendlyError() in live mode
Update server-requirements.md

Typo fix
Merge pull request #2675 from martinduparc/patch-1

Update server-requirements.md
Merge pull request #2650 from Zauberfisch/GridFieldAddExistingAutocompleter

Implove GridFieldAddExistingAutocompleter->doSearch() html output
added filterByCallback() to interface SS_Filterable and all implementing classes
Merge pull request #2626 from Zauberfisch/filterByCallback

filterByCallback added to SS_Filterable interface and implemented in ArrayList