rename confusing getMailHeaderColor to getColorPrimary, ref #3491

apps/encryption/templates/mail.php

apps/federatedfilesharing/settings-personal.php

apps/sharebymail/templates/mail.php

apps/theming/lib/Capabilities.php

apps/theming/lib/Controller/ThemingController.php

apps/theming/lib/IconBuilder.php

apps/theming/lib/Settings/Admin.php

apps/theming/lib/ThemingDefaults.php

apps/theming/tests/CapabilitiesTest.php

apps/theming/tests/Controller/ThemingControllerTest.php

apps/theming/tests/IconBuilderTest.php

apps/theming/tests/Settings/AdminTest.php

apps/theming/tests/ThemingDefaultsTest.php

core/templates/layout.base.php

core/templates/layout.guest.php

core/templates/layout.user.php

core/templates/mail.php

lib/private/legacy/defaults.php

settings/templates/email.new_user.php

themes/example/defaults.php