[1.x] Honour front matter navigation groups set in front matter regardless of subdirectory setting

[caendesilva]

This pull request fixes #1515, which concerns the inconsistency in navigation group behavior when setting navigation groups for standard pages in front matter alongside the dropdown subdirectory setting in the configuration file.

Using the following in _pages/page.md will now cause the page to be put in the appropriate dropdown regardless of the hyde.navigation.subdirectories setting. This better follows the principle of least astonishment as setting the front matter is an explicit instruction to HydePHP.

---
navigation.group: My Group
---

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.97%. Comparing base (f094c1b) to head (37d0ccb).
Report is 1 commits behind head on master.

❗ Current head 37d0ccb differs from pull request most recent head ee7e8a6. Consider uploading reports for the commit ee7e8a6 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1703   +/-   ##
=========================================
  Coverage     99.97%   99.97%           
- Complexity     1742     1745    +3     
=========================================
  Files           181      181           
  Lines          4657     4661    +4     
=========================================
+ Hits           4656     4660    +4     
  Misses            1        1           

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