Add GitHub Action to push source translation strings to Transifex

- requires TX_TOKEN secret to be set on GitHub
- you can generate a token at https://www.transifex.com/user/settings/api/
- `tx push` should fail silently if this is not provided

.github/workflows/transifex.yml

@@ -0,0 +1,39 @@
+# This workflow will push the iD source strings to transifex
+# - requires TX_TOKEN secret to be set on GitHub
+# - you can generate a token at https://www.transifex.com/user/settings/api/
+# - job should fail silently if no token provided
+
+name: transifex
+
+on:
+  push:
+    branches:
+      - develop
+
+jobs:
+  transifex:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: '3.x'
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install transifex-client
+    - name: Create .transifexrc
+      uses: DamianReeves/write-file-action
+      with:
+        path: ${{env.home}}/.transifexrc
+        write-mode: overwrite
+        contents: |
+          [https://www.transifex.com]
+          api_hostname = https://api.transifex.com
+          hostname = https://www.transifex.com
+          password = ${{secrets.TX_TOKEN}}
+          username = api
+    - name: Push source strings to Transifex
+      run: tx push --source --no-interactive