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/config.go

@@ -1768,7 +1768,7 @@ type SupportSettings struct {
 }
 
 func (s *SupportSettings) SetDefaults() {
-	if !IsSafeLink(s.TermsOfServiceLink) {
+	if !isSafeLink(s.TermsOfServiceLink) {
 		*s.TermsOfServiceLink = SupportSettingsDefaultTermsOfServiceLink
 	}
 
@@ -1776,7 +1776,7 @@ func (s *SupportSettings) SetDefaults() {
 		s.TermsOfServiceLink = NewString(SupportSettingsDefaultTermsOfServiceLink)
 	}
 
-	if !IsSafeLink(s.PrivacyPolicyLink) {
+	if !isSafeLink(s.PrivacyPolicyLink) {
 		*s.PrivacyPolicyLink = ""
 	}
 
@@ -1784,7 +1784,7 @@ func (s *SupportSettings) SetDefaults() {
 		s.PrivacyPolicyLink = NewString(SupportSettingsDefaultPrivacyPolicyLink)
 	}
 
-	if !IsSafeLink(s.AboutLink) {
+	if !isSafeLink(s.AboutLink) {
 		*s.AboutLink = ""
 	}
 
@@ -1792,7 +1792,7 @@ func (s *SupportSettings) SetDefaults() {
 		s.AboutLink = NewString(SupportSettingsDefaultAboutLink)
 	}
 
-	if !IsSafeLink(s.HelpLink) {
+	if !isSafeLink(s.HelpLink) {
 		*s.HelpLink = ""
 	}
 
@@ -1800,7 +1800,7 @@ func (s *SupportSettings) SetDefaults() {
 		s.HelpLink = NewString(SupportSettingsDefaultHelpLink)
 	}
 
-	if !IsSafeLink(s.ReportAProblemLink) {
+	if !isSafeLink(s.ReportAProblemLink) {
 		*s.ReportAProblemLink = ""
 	}
 
@@ -3669,7 +3669,7 @@ func (s *ServiceSettings) isValid() *AppError {
 	if host == "" {
 		isValidHost = true
 	} else {
-		isValidHost = (net.ParseIP(host) != nil) || IsDomainName(host)
+		isValidHost = (net.ParseIP(host) != nil) || isDomainName(host)
 	}
 	portInt, err := strconv.Atoi(port)
 	if err != nil || !isValidHost || portInt < 0 || portInt > math.MaxUint16 {
@@ -3974,3 +3974,71 @@ func isTagPresent(tag string, tags []string) bool {
 
 	return false
 }
+
+// Copied from https://golang.org/src/net/dnsclient.go#L119
+func isDomainName(s string) bool {
+	// See RFC 1035, RFC 3696.
+	// Presentation format has dots before every label except the first, and the
+	// terminal empty label is optional here because we assume fully-qualified
+	// (absolute) input. We must therefore reserve space for the first and last
+	// labels' length octets in wire format, where they are necessary and the
+	// maximum total length is 255.
+	// So our _effective_ maximum is 253, but 254 is not rejected if the last
+	// character is a dot.
+	l := len(s)
+	if l == 0 || l > 254 || l == 254 && s[l-1] != '.' {
+		return false
+	}
+
+	last := byte('.')
+	ok := false // Ok once we've seen a letter.
+	partlen := 0
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		switch {
+		default:
+			return false
+		case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '_':
+			ok = true
+			partlen++
+		case '0' <= c && c <= '9':
+			// fine
+			partlen++
+		case c == '-':
+			// Byte before dash cannot be dot.
+			if last == '.' {
+				return false
+			}
+			partlen++
+		case c == '.':
+			// Byte before dot cannot be dot, dash.
+			if last == '.' || last == '-' {
+				return false
+			}
+			if partlen > 63 || partlen == 0 {
+				return false
+			}
+			partlen = 0
+		}
+		last = c
+	}
+	if last == '-' || partlen > 63 {
+		return false
+	}
+
+	return ok
+}
+
+func isSafeLink(link *string) bool {
+	if link != nil {
+		if IsValidHttpUrl(*link) {
+			return true
+		} else if strings.HasPrefix(*link, "/") {
+			return true
+		} else {
+			return false
+		}
+	}
+
+	return true
+}