This commit introduces a new framework for building user tutorials as
popups using the Tippy JS library. Currently, the new framework is used
to replace the old notification spotlight and tips and show a new one
related to the topic timeline.
All popups follow the same structure and have a title, a description and
two buttons for either dismissing just the current tip or all of them
at once.
The state of all seen popups is stored in a user option. Updating
skip_new_user_tips will automatically update the list of seen popups
accordingly.
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