Add person and place document relations pages (#1586)

[blms]

In this PR

Per #1586:

• Add related documents page for people and places
• Use a view mixin and a template snippet to make related code reusable across both people and places
• Add <select>-based mobile navigation for entity detail pages, per designs

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.83%. Comparing base (42a8800) to head (610e2e8).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1587 +/- ## =========================================== + Coverage 98.81% 98.83% +0.01% =========================================== Files 238 238 Lines 13809 13939 +130 =========================================== + Hits 13646 13776 +130 Misses 163 163 ```