Resolve duplicate memberships, superiors/subordinates, and areas of operation

[hancush]

Overview

This PR resolves three bugs that manifested as duplicates in the user interface:

1. Updates the import so that actual duplicate memberships are not created (See Apparent duplicates in superiors/subordinates on person detail #697 (comment))
2. Updates the person view to retrieve a distinct list of organizations to which a person belongs, in order to retrieve superiors and subordinates without duplicating them (Again, see Apparent duplicates in superiors/subordinates on person detail #697 (comment))
3. Updates the sort methodology for areas and emplacements so locations cannot be potentially multiplied due to multiple start/end dates (See Areas of operation are sometimes duplicated on organization detail pages #774 (comment))

Connects #697, #774.

Testing Instructions

• Confirm CI passes.

[hancush]

get_value seems to return an arbitrary object if there is more than one value for a complex field. This mixin adds properties to classes with start and end dates so you can retrieve the earliest start date and latest end date. This is handy for displaying information sorted by either/both dates, else the sort appears nonsensical.

[fgregg]

if role_name can have whitespace in it, seems safest to do the strip before evaluating the truthyness of role_name.

[hancush]

Great call, @fgregg, done for both role and rank in 4a37057.