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:

[Step7750]

We should generally avoid using localStorage within the Steam page since it is untyped and starts to "pollute" a common space for Steam itself and other extensions.

It's preferrable to use extension-specific storage or caching.

[Step7750]

Instead of handling cache breaking in a more manual way, perhaps we should pass in the offer_id of the last element when making the FetchSteamTrades request.

You can then wrap FetchSteamTrades as a CachedHandler, add the trade_offer_id as a request parameter (perhaps along with the Steam of the user on the page), and then you get caching for "free".

[GODrums]

I would really like the CachedHandler to be the solution, but I think can't quite do the job for two reasons:

1. sent / received pages display different trade offers, hence the trade_offer_id would be different
2. the cache is not persistent across browser restarts

I gave it a try in 0d4620b, but I couldn't get it to cache the requests correctly even for the same parameters.