Capture unapplied mainnet tweaks

[Shadowfiend]

We've got a few tweaks that we applied on mainnet in #210 that didn't bubble back to the main branch. Let's make sure these tweaks, or the appropriate adaptations based on the current dApp, land on the main branch. Following is a list.

• [x] Handling mainnet config (https://github.com/keep-network/tbtc-dapp/commit/f1536a829bcd55f59f8fa4f94944d1c604c6dff9). Ideally, this would be a toggle we'd make based on the Ropsten chain id (see #205 for some initial work in this dept). -> see #352
• [x] Clarifying that funding should not happen from an exchange (https://github.com/keep-network/tbtc-dapp/pull/210/commits/ff5c294e6917023d713390cfa9d3da58bf56bf0a). -> see #352
• [x] Call out the exact nature of the expected BTC payment (https://github.com/keep-network/tbtc-dapp/commit/8dffb7d4d5608a1072e9e2f16a13b0ff9007f5d3). -> see #352
• [x] Clarify how many Bitcoin confirmations we are waiting for (https://github.com/keep-network/tbtc-dapp/commit/63dbad8750864ab13e333b35f625de768fef5c6a). This should live in the Confirming component. Ideally we would read the confirmation count from the constants contract, and track how many confirmations we've already observed, but that's likely best served by a separate issue. -> see #353

Remaining commits on that branch have either been obsoleted or applied.