Removing restaurant logic from the backend, attempting to fix relative frontend links, and permanently installing serve

[NickPriv]

• Install serve so that we can easily run npm start locally. (We can also add a dev script which uses react-scripts-start, but I like that serve creates a publicly accessible URL which can be shared, so I haven't added the dev script at this time)
• Fixing links in the LinkButton component so that they go to relative paths instead of localhost:3000. According to a StackOverflow post, if we add // to the start of a link, then it will use a local path instead of directing to localhost:3000. Tested locally, the links still work, but we have to test this on Heroku to verify.
• Shortening the welcome message. Didn't need to add this, but I thought parts of it were redundant.
• Deleted the Restaurants backend logic: Controller, DAL, DALImpl, and model object.

[oxyflush]

Reminder: Need to also delete Restaurants table off of MongoDB.

[NickPriv]

Reminder: Need to also delete Restaurants table off of MongoDB.

This is now done.