Improve utils of slices (#22379)

- Move the file `compare.go` and `slice.go` to `slice.go`.
- Fix `ExistsInSlice`, it's buggy
  - It uses `sort.Search`, so it assumes that the input slice is sorted.
- It passes `func(i int) bool { return slice[i] == target })` to
`sort.Search`, that's incorrect, check the doc of `sort.Search`.
- Conbine `IsInt64InSlice(int64, []int64)` and `ExistsInSlice(string,
[]string)` to `SliceContains[T]([]T, T)`.
- Conbine `IsSliceInt64Eq([]int64, []int64)` and `IsEqualSlice([]string,
[]string)` to `SliceSortedEqual[T]([]T, T)`.
- Add `SliceEqual[T]([]T, T)` as a distinction from
`SliceSortedEqual[T]([]T, T)`.
- Redesign `RemoveIDFromList([]int64, int64) ([]int64, bool)` to
`SliceRemoveAll[T]([]T, T) []T`.
- Add `SliceContainsFunc[T]([]T, func(T) bool)` and
`SliceRemoveAllFunc[T]([]T, func(T) bool)` for general use.
- Add comments to explain why not `golang.org/x/exp/slices`.
- Add unit tests.

routers/web/user/notification.go

@@ -214,7 +214,7 @@ func NotificationSubscriptions(ctx *context.Context) {
 	ctx.Data["SortType"] = sortType
 
 	state := ctx.FormString("state")
-	if !util.IsStringInSlice(state, []string{"all", "open", "closed"}, true) {
+	if !util.SliceContainsString([]string{"all", "open", "closed"}, state, true) {
 		state = "all"
 	}
 	ctx.Data["State"] = state

routers/web/user/setting/profile.go

@@ -413,7 +413,7 @@ func UpdateUserLang(ctx *context.Context) {
 	ctx.Data["PageIsSettingsAppearance"] = true
 
 	if len(form.Language) != 0 {
-		if !util.IsStringInSlice(form.Language, setting.Langs) {
+		if !util.SliceContainsString(setting.Langs, form.Language) {
 			ctx.Flash.Error(ctx.Tr("settings.update_language_not_found", form.Language))
 			ctx.Redirect(setting.AppSubURL + "/user/settings/appearance")
 			return