FEATURE: Lock the UI and present a modal during publish/discard

[grebaldi]

Currently, after clicking on "Publish", "Publish all", "Discard" or "Discard all", the busy-state of the subsequent operation is only reflected right inside the Publishing menu button:

During the operation, the button is disabled and displays a loading indicator. The rest of the UI however remains operable - which is problematic, because editors shouldn't actually be able to edit content while their workspace is being published or discarded.

Success or failure of the operation is communicated to the user via a flash message.

@ahaeslich, @mficzel and I came up with the idea to replace both busy-state indication and flash message with modals that properly lock the UI during the entire operation. That modal context can then later be used as a stage for a dialog workflow for conflict resolution (#3742).

Acceptance criteria

(https://excalidraw.com/#room=98b84c8b6a652784af0a,oiqvD1exm6V8Hu4kttSGdg)

• After clicking on "Publish", "Publish all", "Discard" or "Discard all", a modal appears that indicates the ongoing operation, and locks the UI thus preventing other operations
• Once the Publish/Discard-operation is finished, the user is presented with a modal indicating success or failure of the operation
• The busy-state of Publish/Discard-operations is no longer indicated inside the Publish-button

Nice to have

• During an ongoing Publish/Discard-operation, the user is warned before closing the tab or reloading the page (see also: Prevent closing browser window while isSaving #2050 & FEATURE: Alert if user closes the browser while saving #2059)