Bootstrap's tooltip JS can read the placement from a `data-placement`
attribute.
Further, when we supply the `selector` option to `tooltip`, tooltips
will be added to any dynamically-added elements matching the selector,
without us having to re-call the `tooltip` method.
`ui_blue` wasn't added to the list of classes to remove, so if a user
changed to that theme, any subsequent changes wouldn't be live-updated.
This change refactors Gitlab::Theme a bit to make it harder for this to
happen in the future with new themes.