New: Token for track artist (as opposed to album artist) (#1910)

* New: Token for track artist (as opposed to album artist)

* - Move logic to AddTrackTokens instead of separate AddTrackArtistTokens method
- Just use the first artist name

* Update FIleNameSampleService to handle this token properly

* Fall back to album artist if there's no track artist

* Show tokens in naming modal

frontend/src/Settings/MediaManagement/Naming/NamingModal.js

@@ -82,6 +82,12 @@ const trackTitleTokens = [
   { token: '{Track CleanTitle}', example: 'Track Title' }
 ];
 
+const trackArtistTokens = [
+  { token: '{Track ArtistName}', example: 'Artist Name' },
+  { token: '{Track ArtistNameThe}', example: 'Artist Name, The' },
+  { token: '{Track ArtistCleanName}', example: 'Artist Name' }
+];
+
 const qualityTokens = [
   { token: '{Quality Full}', example: 'FLAC Proper' },
   { token: '{Quality Title}', example: 'FLAC' }
@@ -411,6 +417,28 @@ class NamingModal extends Component {
                     </div>
                   </FieldSet>
 
+                  <FieldSet legend="Track Artist">
+                    <div className={styles.groups}>
+                      {
+                        trackArtistTokens.map(({ token, example }) => {
+                          return (
+                            <NamingOption
+                              key={token}
+                              name={name}
+                              value={value}
+                              token={token}
+                              example={example}
+                              tokenSeparator={tokenSeparator}
+                              tokenCase={tokenCase}
+                              onPress={this.onOptionPress}
+                            />
+                          );
+                        }
+                        )
+                      }
+                    </div>
+                  </FieldSet>
+
                   <FieldSet legend="Quality">
                     <div className={styles.groups}>
                       {