Add "Submit" Button to Upload Form

We'll need to add a submit button to the form to allow users to push their photos after they've selected them and selected the appropriate district competition.

Looking at the Material UI docs, it seems reasonable that we'd be able to use a "LoadingButton" with the "send" icon; something like this:

    <FormControl fullWidth mt={3}>
      <LoadingButton
        type="submit"
        endIcon={<SendIcon />}
        loading={this.state.uploading}
        loadingPosition="end"
        variant="contained"
      >
    </FormControl>

That can all be placed in the frontend/src/PhotoUploadForm.js file between lines 90 and 91 (the "</FormControl>" and "</form>" tags).

This entails adding another React element, as well. We'll need to add uploading to the state structure (lines 15-19 of the same file) and give it a default value of false.