Cleanup model/utils.go (#17945)

* Cleanup model/utils.go

- Removed some functions which were unsued. (The unused linter was somehow failing to catch this).
- Moved some functions inside other packages.
- Removed two duplicate instances of the same function - RemoveDuplicateStrings and UniqueStrings.
- Moved some regexes to global vars to prevent compilation every time.

```release-note
NONE
```

* fix lint

```release-note
NONE
```

* bring back unused exception

```release-note
NONE
```

* fix tests

```release-note
NONE
```

* Address review comments

```release-note
NONE
```

model/user.go

@@ -931,30 +931,6 @@ func CleanUsername(username string) string {
 	return s
 }
 
-func IsValidUserNotifyLevel(notifyLevel string) bool {
-	return notifyLevel == ChannelNotifyAll ||
-		notifyLevel == ChannelNotifyMention ||
-		notifyLevel == ChannelNotifyNone
-}
-
-func IsValidPushStatusNotifyLevel(notifyLevel string) bool {
-	return notifyLevel == StatusOnline ||
-		notifyLevel == StatusAway ||
-		notifyLevel == StatusOffline
-}
-
-func IsValidCommentsNotifyLevel(notifyLevel string) bool {
-	return notifyLevel == CommentsNotifyAny ||
-		notifyLevel == CommentsNotifyRoot ||
-		notifyLevel == CommentsNotifyNever
-}
-
-func IsValidEmailBatchingInterval(emailInterval string) bool {
-	return emailInterval == PreferenceEmailIntervalImmediately ||
-		emailInterval == PreferenceEmailIntervalFifteen ||
-		emailInterval == PreferenceEmailIntervalHour
-}
-
 func IsValidLocale(locale string) bool {
 	if locale != "" {
 		if len(locale) > UserLocaleMaxLength {