Add playlist creation from add dialog

[daveliu]

Summary

• Add an inline create-playlist form to the Add to Playlist dialog.
• Preserve the selected song_id when creating a new playlist from that dialog.
• Create the playlist and add the selected song in one transaction, then redirect to the new playlist.

Closes #464

Validation

• bin/rails test test/controllers/playlists_controller_test.rb:22 test/controllers/dialog/playlists_controller_test.rb:6
• bundle exec rubocop app/controllers/playlists_controller.rb test/controllers/playlists_controller_test.rb test/controllers/dialog/playlists_controller_test.rb
• bundle exec erb_lint app/views/dialog/playlists/index.html.erb

Note: the broader focused controller suite hit an unrelated missing test asset (application.css) in this disposable Docker test environment when rendering the playlist index.

[daveliu]

Chrome verification for the Add to Playlist dialog on this branch:

• Opened the Songs page in Google Chrome
• Opened the Add to Playlist modal from the song list flow
• Confirmed the new Create Playlist field is present
• Confirmed the Create and Add submit button is present
• Confirmed existing playlist targets still render below it

[aidewoode]

@daveliu Thank you for your PR. because the redesign I am working on will have conflicts on your change. I will hold on this PR until my redesign is finished