Show whether an option is already mapped in the selector

* adds `join_mapping` to ModalSelector options
* adds popover to "Mapped (ext)" text in selectors
* enables StickyPopover to take content from direct children of the
  element containing the `data-toggle` attribute (which allows
  easier live-binding than attribute content)
* adds mustache helper `if_result_has_direct_mappings` and fixes
  `if_result_has_extended_mappings`
Merge pull request #655 from reciprocity/feature/show-already-mapped-objects

Show whether an option is already mapped in the selector