Split out is_changing_upload from is_submitting (#9536)

There is no reason to disable the composer textarea when some media metadata
is being modified, nor is there any reason to focus the textarea when some
media metadata has been modified (prevents clicking one image's description
field right after having modified another).

app/javascript/mastodon/features/compose/containers/compose_form_container.js

@@ -22,6 +22,7 @@ const mapStateToProps = state => ({
   caretPosition: state.getIn(['compose', 'caretPosition']),
   preselectDate: state.getIn(['compose', 'preselectDate']),
   is_submitting: state.getIn(['compose', 'is_submitting']),
+  is_changing_upload: state.getIn(['compose', 'is_changing_upload']),
   is_uploading: state.getIn(['compose', 'is_uploading']),
   showSearch: state.getIn(['search', 'submitted']) && !state.getIn(['search', 'hidden']),
   anyMedia: state.getIn(['compose', 'media_attachments']).size > 0,