SubmissionsController: provide helpful redirects when invalid URLs are accessed

[lizheym]

What does this PR do?

This PR updates the SubmissionsController to add an index action and update the new action.

• In the case of the index action, we redirect to the proposals #index route and display a notice flash message.
• In the case of the new action, if there is no proposal provided, we redirect to the proposals #index route and display a notice flash message.

Why was this work done? Is there a related Issue?

This work was done in response to Issue Number 48, which notes that if someone happens to alter the URL (likely with knowledge of rails routes) to try to view the submissions index/new pages, an unhelpful error page is shown. The flash message should help contextualize the role of a submission, articulating why these pages do not exist (or need to exist).

Are there any manual testing steps?

• Navigate to http://localhost:3000/submissions/new
• Navigate to http://localhost:3000/submissions

Caveat about styling

This related CR by Hilary styles the flash messages, so they will look much better than shown below!!

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Screenshots

http://localhost:3000/submissions/new

http://localhost:3000/submissions

[nodunayo]

Great stuff! I'm going to change the wording of the flash messages slightly, but other than that: awesome! Thank you!