Dashboard: Support configuring default timezone via config file (#27404)

Add a default timezone that the administrator can set in the settings. 
This setting is be used as default for the users timezone preference.
Can be used when creating Grafana instances without administrator 
intervention, in order to give user the correct default timezone.

Fixes #25654

pkg/setting/date_formats_test.go

@@ -0,0 +1,37 @@
+package setting
+
+import (
+	"testing"
+
+	"gopkg.in/ini.v1"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestValueAsTimezone(t *testing.T) {
+	tests := map[string]struct {
+		output string
+		hasErr bool
+	}{
+		"browser":          {"browser", false},
+		"UTC":              {"UTC", false},
+		"utc":              {"browser", true},
+		"Amsterdam":        {"browser", true},
+		"europe/amsterdam": {"browser", true},
+		"Europe/Amsterdam": {"Europe/Amsterdam", false},
+	}
+
+	sec, err := ini.Empty().NewSection("test")
+	assert.NoError(t, err)
+	key, err := sec.NewKey("test", "")
+	assert.NoError(t, err)
+
+	for input, expected := range tests {
+		key.SetValue(input)
+
+		output, err := valueAsTimezone(sec, "test", "default")
+
+		assert.Equal(t, expected.hasErr, err != nil, "Invalid has err for input: %s err: %v", input, err)
+		assert.Equal(t, expected.output, output, "Invalid output for input: %s", input)
+	}
+}