Fix code scanning alert - Missing rate limiting

[Bellisario]

Tracking issue for:

• https://github.com/Bellisario/node-snapdrop/security/code-scanning/4

Should be introduced ASAP a fix for this vulnerability since an user could use DDoS attacks to shut down the entire application just by requesting index.html.

I think I'll fix this using express-rate-limit.

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This is not a really high severity vulnerability since Heroku hosting has an internal DDoS attack protection, but this is really bad instead for users hosting their own node-snapdrop server.