Added moderation support

[ishevche]

It resolves issue #1.

Every listing, before being sent to a specified channel, is sent to a moderator, defined by its CHAT_ID in the .env file.

For each listing, a moderator has two options: approve and decline. If the moderator approves the listing, it is posted to the channel, and the seller is notified with a message containing a button to revoke the listing. If the listing is declined, only a notification for the seller is sent.

Six newly created text variables are used:

• bot.listing_approved{listing} - notification for seller with listing approval
• bot.listing_declined{listing} - notification for seller with listing approval
• bot.moderator_approval - text displayed on the approve button for the moderator
• bot.moderator_declination - text displayed on the decline button for the moderator
• bot.approved_successfully - text contained in approving callback_query answer
• bot.declined_successfully - text contained in declining callback_query answer
• bot.send_to_moderation - replacement for thanks_sale string, sent to a user when a newly created listing is sent to a moderator for check

The first two variables accept listing - a text of the message displayed in the channel.

[Salz0]

Hey man, do you plan on working this PR anymore?

[Salz0]

Great, thank you man for this big PR and for sticking to it util it's done. Highly appreciated 🙌