Set up Transifex for translations

[barbeau]

Summary:

OTSF now has a Transifex account that is actively being used for translations on iOS:
https://www.transifex.com/open-transit-software-foundation/

We should do the same on Android.

Steps to reproduce:

Try to do a translation

Expected behavior:

Make it easy via Transifex

Observed behavior:

It's hard

[barbeau]

I set up the push of strings to Transifex in a GitHub Action as part of this project - barbeau/gpstest#526 (comment).

It uses a script:
https://github.com/barbeau/gpstest/blob/master/scripts/push-to-transifex.sh

...that's triggered from the GitHub Action:
https://github.com/barbeau/gpstest/blob/master/.github/workflows/transifex.yml

Here's the magic for authentication:

export TX_TOKEN=$TRANSIFEX_API_TOKEN
tx push -s --force --no-interactive

and this in the GitHub Action:

env:
  TRANSIFEX_API_TOKEN: ${{ secrets.TRANSIFEX_API_TOKEN }}

Apparently the TRANSIFEX_API_TOKEN GitHub secret for Transifex API token has to be set up as a "Repository" secret and not "Environmental" secret in the GitHub UI.

[barbeau]

OBA iOS has this step automated too, but configured via Transifex to pull strings on GitHub PR merges. Might be a cleaner approach that doesn't require scripts.