example-dapp: Update to support single-asset balances.

[Shaptic]

Related: stellar/go#4723. This is split off into a separate issue for better tracking.

Things to be done:

• [x] Investigation:
  • As part of stellar/soroban-tools#325, the token create subcommand is removed.
  • The dApp uses this to create its EXT token in initialize.sh.
  • It also interacts with that token via the approve() and mint() functions.
  • These need to be replaced with the Classic equivalents:
    • creation is now issuing an asset, so basically a no-op
    • both the user and the contract need to establish a trustline to the new asset
    • the approve() should now use incr_allow() based on the new token contract
    • minting is now just a payment
• [x] Implementation:
  • [x] Replace the token creation with token wrapping.
  • [x] Establish trustlines to the issuer from the wallet.
  • [x] Coalesce all customizable decimal references to be 7 (all Stellar assets use this value).
  • [x] Update the token contract spec WASM file.
  • [x] Update token usage to the latest interface
  • [x] Update crowdfunding contract test to interact with the new token interface. Moved to #81.
  • [x] Replace minting with a payment operation
• [x] Testing:
  • [x] Ensure everything runs with the new quickstart image, soroban-tools and js-soroban-client releases
  • [x] Ensure the new initialization works
  • [x] Ensure minting works, incl. decimal precision
  • [x] Ensure approvals and deposits work
• [x] Documentation: Dropped sandbox instructions from the README, since we need classic transactions now (which ledger.json doesn't support).