Bugfix - Enforce URL length check in LinkMetadata before attempting to insert to database (#29589)

* Add missing URL length validation in LinkMetadata

* Added MaxURLLength constant

* Add test to validate new URL length limit

* Add comment about table constraint

* Use a seperate error string for URL length errors

* Added error string fpr URL length

* Updated error message

* Update en.json

* Fixed gofmt error in link_metadata.go

* use right txn id

```release-note
NONE
```

---------

Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/link_metadata_test.go

@@ -146,6 +146,16 @@ func TestLinkMetadataIsValid(t *testing.T) {
 			},
 			Expected: false,
 		},
+		{
+			Name: "should be invalid because of URL length being too long",
+			Metadata: &LinkMetadata{
+				URL:       "http://example.com/?" + strings.Repeat("a", 2048),
+				Timestamp: 1546300800000,
+				Type:      LinkMetadataTypeImage,
+				Data:      &PostImage{},
+			},
+			Expected: false,
+		},
 	} {
 		t.Run(test.Name, func(t *testing.T) {
 			appErr := test.Metadata.IsValid()