purple: improve environment handling

This commit improves the handling of different Purple environments:

- 3 environments were added (test, staging, production) to fulfill all
  testing needs

- Damus website constants were also added (This will become important
  for the next few commits)

- Toggle settings were replaced with a picker, where the user can select
  one of the 3 environments (test, staging, production)

- Damus purple page and website address links can now be obtained
  directly from the DamusPurple struct, which improves flexibility and
  reduces complexity for code that consumes these constants.

Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Reviewed-by: William Casarin <jb55@jb55.com>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Views/Purple/DamusPurpleView.swift

@@ -412,7 +412,7 @@ struct DamusPurpleView: View {
                     Spacer()
                     Link(
                         NSLocalizedString("Learn more", comment: "Label for a link to the Damus Purple landing page"),
-                        destination: damus_state.settings.purple_api_staging ? Constants.PURPLE_LANDING_PAGE_TEST_URL : Constants.PURPLE_LANDING_PAGE_PRODUCTION_URL
+                        destination: damus_state.purple.environment.purple_landing_page_url()
                     )
                         .foregroundColor(DamusColors.pink)
                         .padding()