Frontend cleanup, layout improvements, progress indicator, file uploads, virtualized songs view

Closes #19 and #17

See merge request unicorns/bmbf!10

For the left and right positioning use `marginLeft`/`marginRight` `'auto'`

Improves the item render performance

Workaround for Masonic in production builds

Only cosmetic change for use on the computer

!7 introduced changes that were not considered by !8

TODO: Sometimes a bit wonky on chrome.