Add permanent 2.03 support

[hayfield]

• 262 noted that pyIATI needs to support 2.03.

• 272 added initial support in a non-sustainable manner.

• In-line with the suggestions in #272, the Version changes (#223) have occurred based on a 2.02 branch.

• 295 reverts the temporary addition of 2.03.

2.03 now needs re-adding in a sustainable manner based on these changes.

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• [x] Add 2.03 as a version in the constants defining known versions
• [x] Update tests with a latest_version mark
• [x] Add 2.03 content and test data
  • [x] Schemas
  • [x] Activity
  • [x] Org
  • [x] Common
  • [x] XML
  • [x] Registry Record
  • [x] Ruleset
  • [x] Codelists
  • [x] Codelist Mapping File (double check it has all the required fixes!)
  • [x] Test data
• [x] Perform NE Codelist migration
  • [x] Update symlinks
  • [x] Ensure symlinks are relative rather than absolute
  • [x] Update Codelist counts
• [x] Update documentation and Changelog to detail that 2.03 is now added in a more sustainable manner that is better known to be correct
  • [x] Re-add reverted documentation from 15d59957502815ac4f3faf105c6811a4f4a6d562
  • [x] Readme
  • [x] Changelog