This reverts commit a3a2a4102d.
#10046 causes a crash on save. MainPage::UpdateSettings() is unable to update the navigation view's selected item due to an "incorrect parameter". This is particularly strange to see because #10046 only modifies the navigation view's header, not the menu items themselves. Reverting this change fixes that crash (verified).
Reopens#9694
## Summary of the Pull Request
Adds the profile icons to the page header. I had to manually create the header, and manually bind it to the `Icon` and `Content` of each `NavViewItem`.
It's important that each `NavViewItem`'s icon is set as an `IconSource`, so that we can bind to it. If it's just a plain old `FontIcon`, then we can't re-use it.
Additionally, I removed the manual sizing of all font icons to font size 12. That would make font icons _tiny_ in the header. Now, they'll properly re-use the size of the `NavigationViewTitleHeaderContentControlTextStyle` in the nav view header. This involved also manually making the icons smaller on the `AddProfile` page and in the `CommandPalette`.
As per usual, images are in Teams
## PR Checklist
* [x] Closes#9694
* [x] I work here
* [ ] Tests added/passed
* [n/a] Requires documentation to be updated
## Validation Steps Performed
* Checked (bitmap|font) icons in tabs
* (bitmap|font) icons in the flyout
* (bitmap|font) icons in command palette
* (bitmap|font) icons in the nav view
* (bitmap|font) icons in the header
* (bitmap|font) icons in the add profile page
