Style destructive actions destructively
Feature to allow that changing the SSTemplateParser through the Injector system

The motivation for this was to allow module developers to change what parser is used to parse SilverStripe templates.
This change enables people to compile their own version of the SilverStripe template parser and use it without modifying core files.
GridField 'Add existing' action styling improvements
Email fluent API regression (fixes #2581)
Merge pull request #2582 from kinglozzer/2567-add-existing-gridfield-styling

GridField 'Add existing' action styling improvements (fixes #2567)
Merge pull request #2557 from camspiers/template-parser-configurable

Feature to allow changing the SSTemplateParser used in SSViewer
Merge pull request #2520 from wilr/destructivestyles

Style destructive actions destructively
FIX: Move stage choosing into a pre-request filter.

This ensures that the correct stage is selected, even if the request
does not come through the model as controller system. This fixes an
issue where custom controllers would always be on the "Stage" stage.
Merge pull request #2585 from ss23/3.1-staging-fix

FIX: Move stage choosing into a pre-request filter.
MINOR typo where display_errors wasn't checked properly
Adjust UploadField edit form size

Thanks @kinglozzer for getting this started.
Fixes https://github.com/silverstripe/silverstripe-cms/issues/878
Replaces https://github.com/silverstripe/silverstripe-framework/pull/2221