Automatic funding fix

[RiccardoM]

Description

This PR moves the check and request for funding from the sync use case to the remote chain source. In this way, each transaction will trigger the faucet to send the user more funds if needed (fixes #84).

Checklist

• [x] Targeted PR against correct branch.
• [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• [x] Wrote unit tests.
• [x] Run flutter format.
• [ ] Updated the documentation.
• [x] Added an entry to the CHANGELOG.md file.
• [ ] Re-reviewed Files changed in the Github PR explorer.

[codecov[bot]]

Codecov Report

Merging #85 into master will increase coverage by 0.24%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   17.54%   17.78%   +0.24%     
==========================================
  Files         258      258              
  Lines        6252     6268      +16     
==========================================
+ Hits         1097     1115      +18     
+ Misses       5155     5153       -2     

Impacted Files	Coverage Δ
lib/usecases/posts/usecase_sync.dart	0.00% <0.00%> (ø)
lib/sources/chain/chain_source_impl.dart	83.60% <88.46%> (+2.52%)	:arrow_up:
...b/sources/user/remote/remote_user_source_impl.dart	29.03% <100.00%> (-5.26%)	:arrow_down:

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