Migrates the contribution amounts from old USD setting to new BAT setting

[bsclifton]

Fixes #11719

Auditors: @NejcZdovc, @evq, @petemill

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

• Use an older version of Brave (0.18.36 for example)
• Enable payments and set contribution amount as one of the original amounts (like $10 for example)
• Run a build with this fix in it
• In preferences#payments, you should notice it should be 25 BAT if USD was $5, 50 if $10, 75 if $15, 100 if $20
• Exit Brave
• Open the ledger-state.json file and inspect, looking for properties.fee.amount
• This should have the new value (25, 50, 75, or 100) not the old value (5, 10, 15, 20)

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #11722 into master will increase coverage by 0.1%.
The diff coverage is 88.23%.

@@            Coverage Diff            @@
##           master   #11722     +/-   ##
=========================================
+ Coverage   52.01%   52.12%   +0.1%     
=========================================
  Files         269      269             
  Lines       25534    25553     +19     
  Branches     4069     4074      +5     
=========================================
+ Hits        13282    13319     +37     
+ Misses      12252    12234     -18

Flag	Coverage Δ
#unittest	52.12% <88.23%> (+0.1%)	⬆️

Impacted Files	Coverage Δ
app/browser/api/ledger.js	32.41% <88.23%> (+1.57%)	⬆️
js/actions/appActions.js	17.11% <0%> (+0.6%)	⬆️

[NejcZdovc]

++ looks good to me

[NejcZdovc]

master b27ca58
0.21 2c4c72b
0.20 a42cac1
0.19 c211947