Add ui.explore settings to control view of explore pages (2) (#14094)

Add ui.explore settings to control view of explore pages (2) (#14094)

This is an alternative PR to #13687.

Add `[ui.explore]` settings to allow restricting the
explore pages to logged in users only and to disable the users explore page.

The two proposed settings are:

- `REQUIRE_SIGNIN_VIEW`: Only allows access to the explore pages if the
user is signed in. Also restricts
  - `/api/v1/user/search`
  - `/api/v1/users/{username}`
  - `/api/v1/users/{username}/repos`
  - but does not restrict `/api/v1/users/{username}/heatmap`
- `DISABLE_USERS_PAGE`: Disables the /explore/users page

Fix #2908

Close #13687

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>