Display Metadata on /tradeoffers-pages

[GODrums]

Motivation

Closes #143.

Description

This PR adds the well-known enhancements from tradeoffer-popouts to the /tradeoffers-overview:

• FetchSteamTrades-handler to fetch the data from the Steam API through the access token
• trade_offers.ts page script:
  • Calculates and stores the api data in an accessible format in the DOM
  • Utilizes the localStorage as cache to avoid refetching from the API if there are no new trades since the last fetch
• TradeOfferHolderMetadata Lit-component, which reads the required properties from the DOM
• Removes the check regarding a CSFloat account for the "offer tracking on csfloat"-button. Since all Steam-API features rely on the same host permission, enabling the "offer tracking" is a mandatory pre-condition

Screenshots

Received tradeoffer example:

Sent tradeoffer example: