Remove some instances of "starter kit"

Rename some files and change some identifiers to be neutral to the
application name. This makes it simpler to change everything to a proper
name when cloning this project.

Document in the README how to find the remaining places to change.

Closes #20

test/check-application

@@ -13,7 +13,7 @@ sys.path.append(os.path.join(os.path.dirname(TEST_DIR), "bots/machine"))
 import testlib
 
 
-class TestStarterKit(testlib.MachineCase):
+class TestApplication(testlib.MachineCase):
     def testBasic(self):
         b = self.browser
         m = self.machine

test/vm.install

@@ -1,6 +1,6 @@
 #!/bin/sh
 # image-customize script to enable cockpit in test VMs
-# The starter-kit RPM will be installed separately
+# The application RPM will be installed separately
 set -eu
 
 # don't force https:// (self-signed cert)