Allow multiple modals to be opened

[ffont]

There are a number of things on the new UI that are done using modals: bookmarking sounds, showing sound downloads, user followers... However, if there is a button placed inside a modal which should trigger another modal, this causes problems because we only use one html element for the "generic modal" which gets replaced when a new modal is opened (and there is no way to go "back"). There should be a solution for that. I think the easiest solution is to not replace the single "generic modal" element with the contents of a new modal when this is loaded, but simply add the new modal as a child of some parent element, make sure that the last modal is visible at the top, so when the last is closed, the previous becomes visible. If user interaction is not clear in this way, maybe we should add some other indicator to make clear that multiple modals have been opened.