Clears table after contribution

[NejcZdovc]

Resolves brave/brave-browser#1501

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).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

• clean profile
• enable rewards
• claim grant
• visit some sites so that they are added to the table
• wait for contribution (if you are developer, you can change this in native lib https://github.com/brave-intl/bat-native-ledger/blob/master/src/bat_client.cc#L183)
• make sure that table is cleared and reconcile stamp is updated

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[jasonrsadler]

Good job. One problem I had was that contributions would go through and BAT was deducted even if no sites were listed in auto-contribute table. Will create follow up.

[jasonrsadler]

master: bd5e9f8
0.55: 348bf12
0.56: 9c4d64a