feat: add first_block to the to_json_extended method of base transaction class

[alexruzenhack]

Motivation

Wallet Mobile needs the ability to show a nano contract transaction "status" in the token's transaction details modal, but this "status" requires to know if a nano contract transaction was already confirmed by a block of not, thus the first_block as a first citizen of a transaction.

Acceptance Criteria

• It should add first_block to the to_json_extended to BaseTransaction for transactions.
• It should update documentation in the following resources:
  • /dashboard_tx
  • /transaction
  • /thin_wallet/address_history
  • /thin_wallet/token_history

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.90%. Comparing base (256739a) to head (cbd4c2d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1003      +/-   ##
==========================================
- Coverage   84.92%   84.90%   -0.03%     
==========================================
  Files         298      298              
  Lines       22925    22928       +3     
  Branches     3463     3464       +1     
==========================================
- Hits        19470    19467       -3     
- Misses       2768     2771       +3     
- Partials      687      690       +3     

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