Add permanent 2.03 support

[hayfield]

• pyIATI needs to support 2.03 #262 noted that pyIATI needs to support 2.03.
• Add initial 2.03 support #272 added initial support in a non-sustainable manner.
• In-line with the suggestions in Add initial 2.03 support #272, the Version changes (Improve version parameter #223) have occurred based on a 2.02 branch.
• Revert "Add initial 2.03 support" into integration branch #295 reverts the temporary addition of 2.03.

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

---

• Add 2.03 as a version in the constants defining known versions
• Update tests with a latest_version mark
• Add 2.03 content and test data
  • Schemas
    • Activity
    • Org
    • Common
    • XML
    • Registry Record
  • Ruleset
  • Codelists
  • Codelist Mapping File (double check it has all the required fixes!)
  • Test data
• Perform NE Codelist migration
  • Update symlinks
  • Ensure symlinks are relative rather than absolute
  • Update Codelist counts
• Update documentation and Changelog to detail that 2.03 is now added in a more sustainable manner that is better known to be correct
  • Re-add reverted documentation from 15d5995
  • Readme
  • Changelog