Improved mail header and introduced localized decimal values

[lars-devs]

Hi! I'd like to PR two changes:

First concerns including the field "Date" in the mail nofitication. smtplib doesn't automatically default to the current time. Depending on which provider and e-mail client is used with TGTG scanner, e-mails get either displayed weird and/or labeled as Spam.

Posteo.de requires the e-mail to include Date:

Thunderbird falls back to the current time for all e-mails that miss "Date" in the header (see this post), so it displays all e-mails ever received by TGTG scanner as today.

Second change introduces optional formatting of price, value, and rating. I decided to use babel other than locale, so that the project depends on Python modules only. Using de_DE as locale and EUR as currency, this gets replaced:

price: 3.00 => price_localized: 3,00 €
rating: 4.7 => rating_localized: 4,7
value: 6.00 => value_localized: 6,00 €

Pull Request Checklist

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Did you make your Pull Request on the dev branch?
• Does your submission pass tests? make test
• Have you lint your code locally prior to submission? make lint
• Could you build and run the docker images successfully? make images
• Could you create a running executable? make executable
• Have you added an explanation of what your changes do and why you'd like to include them?
• Have you written new tests for your changes, as applicable?
• updated test for SMTP notifier.
• Have you successfully ran manual tests with your changes locally?