Update i18n scripts to use Transifex REST API v3

[silvanocerza]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Updates the i18n commands.

• What is the current behavior?

i18n commands to push and pull translation files use the deprecated Transifex API 2.0.

• What is the new behavior?

The i18n commands have been updated to use the new Transifex API 3.0.

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

Closes #1494.

After this PR is merged we'll need to update the following repository secrets:

• TRANSIFEX_ORGANIZATION
• TRANSIFEX_PROJECT
• TRANSIFEX_RESOURCE
• TRANSIFEX_API_KEY

---

See how to contribute