Wizard: rm pem validation, add cert warning

Since we support pem, cer, and der, we'd have to validate against each
of these on the frontend. Let's just check that the file is not empty,
and leave this upto users. On top of that, concatenated certificates are
supported, validating that would be too much. This commit also switches
token expiration error into a warning.

src/Components/CreateImageWizard/steps/Registration/SatelliteRegistration.tsx

@@ -88,7 +88,7 @@ const SatelliteRegistration = () => {
                 ? 'Must be a .PEM/.CER/.CRT file no larger than 512 KB'
                 : validated === 'error'
                 ? stepValidation.errors['certificate']
-                : 'Drag and drop a file or upload one'}
+                : 'Drag and drop a valid certificate file or upload one'}
             </HelperTextItem>
           </HelperText>
         </FormHelperText>

src/Components/CreateImageWizard/steps/Registration/components/SatelliteRegistrationCommand.tsx

@@ -46,6 +46,7 @@ const SatelliteRegistrationCommand = () => {
         placeholder="Registration command"
         stepValidation={stepValidation}
         fieldName="command"
+        warning={stepValidation.errors.expired}
       />
       <FormHelperText>
         <HelperText>