SubCategoryPage: use "Back" text for back button label

Nate Grahamrequested to merge
work/ngraham/moar-back-button into master

Right now the back button's label is the name of the category. This presents two problems:

  1. The laws of grouping and proximity suggest that clicking the button will take you back to a category by that name, which isn't the case
  2. The category name might match the name of the page; then the UI shows many copies of the page

To address those issues, make it a more standard "Back" button.

Here's a particularly egregious example with the "Application Permissions" category/KCM (which could benefit from additional fixes beyond the scope of this MR):

Before After
Before After

@teams/usability @teams/vdg

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