R.I.P. Cnchi — Calamares is now the primary installer

- Main install button now launches calamares-next instead of cnchi
  - Button label changed from 'Launch Cnchi Installer' to
    'Install Antergos NeXT'
  - Removed 'Alternative Installer' (Calamares) button — no longer
    needed since Calamares is the only installer
  - Welcome song path updated: /usr/share/cnchi-memes/ →
    /usr/share/antergos-next-memes/ (memes package repurposed)
  - Release notes page: added Cnchi retirement notice with eulogy
  - Readme page: updated instructions, removed cnchi-next link

src/antergos_welcome.py

@@ -250,7 +250,7 @@ class Hello(Gtk.Window):
         self.setup_mute_button()
 
     def play_welcome_song(self):
-        song = "/usr/share/cnchi-memes/its-raining-tacos.opus"
+        song = "/usr/share/antergos-next-memes/its-raining-tacos.opus"
         self.mpv_process = None
         if os.path.isfile(song):
             try:
@@ -436,8 +436,6 @@ class Hello(Gtk.Window):
         """Event for differents actions."""
         name = action.get_name()
         if name == "install":
-            subprocess.Popen(["sudo", "-E", "cnchi"])
-        elif name == "calamares":
             subprocess.Popen(["sudo", "-E", "calamares-next"])
         elif name == "autostart":
             self.set_autostart(action.get_active())