Refactor the setting to make unit test easier (#22405)

Some bugs caused by less unit tests in fundamental packages. This PR
refactor `setting` package so that create a unit test will be easier
than before.

- All `LoadFromXXX` files has been splited as two functions, one is
`InitProviderFromXXX` and `LoadCommonSettings`. The first functions will
only include the code to create or new a ini file. The second function
will load common settings.
- It also renames all functions in setting from `newXXXService` to
`loadXXXSetting` or `loadXXXFrom` to make the function name less
confusing.
- Move `XORMLog` to `SQLLog` because it's a better name for that.

Maybe we should finally move these `loadXXXSetting` into the `XXXInit`
function? Any idea?

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: delvh <dev.lh@web.de>

modules/highlight/highlight.go

@@ -41,12 +41,8 @@ var (
 // NewContext loads custom highlight map from local config
 func NewContext() {
 	once.Do(func() {
-		if setting.Cfg != nil {
-			keys := setting.Cfg.Section("highlight.mapping").Keys()
-			for i := range keys {
-				highlightMapping[keys[i].Name()] = keys[i].Value()
-			}
-		}
+		highlightMapping = setting.GetHighlightMapping()
+
 		// The size 512 is simply a conservative rule of thumb
 		c, err := lru.New2Q(512)
 		if err != nil {