1257 allow selection of units in select metadata menu

[juzen2003]

• Fixes #1257 (implementation in select metadata menu)
• Were any Django, import pipeline, or table_schema files modified? N
  • If YES:
  • Database used: opus3_test_220906
  • All Django tests pass: Y
  • FLAKE8 run on modified code: Y
• Were any JavaScript or CSS files modified? Y
  • If YES:
  • JSHINT run on all affected files: Y
  • Tested on Chrome, Firefox, Safari, Edge, and iOS: Y (Remember to test all browser sizes)
  • Tested for race conditions (with delayed API calls if applicable): NA
• Was the documentation reviewed for necessary changes or additions? Y
  • About
  • Getting Started
  • FAQ
  • API Guide
  • Tooltips

Description of changes:

• Add units with dropdown in selected metadata fields when:
  • The select metadata menu is open
  • A new field is added
    • Store current displayed unit and available units in the data attributes for each menu item so that later on when a new field is added, we can use these info to attached the unit dropdown to that specific field
• Style the dropdown
  • Set the max height
  • Enable scrolling
• Add the dropdown behaviors
  • Scroll on dropdown won't scroll the selected metadata field
  • Click dropdown item will:
    • Close the dropdown
    • Change the displayed unit
    • Update the slug in opus.prefs.cols
      • Add ":" to specify unit, default unit will be used if it's not specified.
      • Check ":" when parsing slugs from opus.prefs.cols.
    • Avoid page refreshing
• Click "Save Changes" will update the metadata values based on selected units

[rfrenchseti]

Also need to add a brief mention of this functionality in the Getting Started Guide. Add a new sentence after this:

"Which metadata fields are displayed in the table view and slideshow view can be changed by clicking on Select Metadata at the top. Fields may be added or removed by clicking on them on the left or removed by clicking on the on the right, and may be reordered by dragging."

First of all fix the existing typo: "clicking on THEM on the right"

Then add: "If available, the units a metadata field is displayed in can be changed by clicking on the current units name shown in parentheses."

The next sentence begins "The fields selected here also affect". Change to "The fields and units selected here also affect"

[juzen2003]

• Close units dropdown when:
  • select metadata modal is closed
  • drag any selected metadata fields
  • click on trash icon of the selected metadata fields
• Restore the display unit on selected metadata fields when the changes are not saved and modal is closed.