Add an optional [community] param to /buy and /sell commands

[qustavo]

I publish my orders in two communities, the default and a currency specific, so when it comes to create an order (buy or sell) I create one, switch community and the copy and paste it. This process is annoying and error prone.

I would be ideal if the order commands allows an optional [community], which when set instruct the bot to also publish the order to such community (along with the one that is currently set by the user).

Something like:

/sell <monto en sats> <monto en fiat> <código fiat> <método de pago> [prima/descuento] [community]

[Matobi98]

Why we're keeping single-destination publishing (one command per community)

After reviewing the workflow requested in #877, we decided not to implement publishing the same order to two communities in a single command.

Making one /buy or /sell call fan out to both the currently selected community and an optional extra one would mean:

• Creating and tracking multiple orders per command.
• Duplicating the whole publish / validation / fee flow (each community has its own fee, currencies, and ban list).
• Complicating cancellation and the overall order lifecycle.

That added complexity isn't worth it for this feature. Instead, this PR keeps the flow simple and explicit through the optional [community] argument.

How it works

1. Publish to your default community — no extra arguments

If you have a default community set (via /setcomm), just run the command as usual and it publishes there:

/sell 100 20 USD "bank transfer"
/buy 100 20 USD "bank transfer"

2. Publish to another community — pass the extra arguments

Run the command again, this time adding the price margin and the target community (its @group handle or Telegram group id). The community is positional, so the price margin slot must come first:

/sell 100 20 USD "bank transfer" 0 @testcommunity
/buy 100 20 USD "bank transfer" 0 -1001234567890

So publishing to two communities is fully supported — it's just two explicit commands instead of one implicit fan-out. Each order stays tied to a single, clearly-chosen destination, and we avoid the extra complexity of multi-destination orders.

Scope

Given this, the PR delivers the optional-community parameter but does not fully close #877 as originally framed. We're realigning the scope accordingly — Refs #877 rather than Closes #877.