Support serving of backend translations (#12453)

* Add view to support backend translation fetching

* Load backend translations from component json

* Translations for season sensor

* Scripts to merge and unpack Lokalise translations

* Fix copy paste error

* Serve post-lokalise translations to frontend

* Linting

* Auto-deploy translations with Travis

* Commit post-lokalise translation files

* Split logic into more helper functions

* Fall back to English for missing keys

* Move local translation copies to `.translations`

* Linting

* Initial tests

* Remove unnecessary file check

* Convert translation helper to async/await

* Convert translation helper tests to async/await

* Use set subtraction to find missing_components

* load_translation_files use component->file mapping

* Remove duplicated resources fetching

Get to take advantage of the slick Python 3.5 dict merging here.

* Switch to live project ID

tests/testing_config/custom_components/switch/.translations/test.de.json

@@ -0,0 +1,6 @@
+{
+  "state": {
+    "string1": "German Value 1",
+    "string2": "German Value 2"
+  }
+}

tests/testing_config/custom_components/switch/.translations/test.en.json

@@ -0,0 +1,6 @@
+{
+  "state": {
+    "string1": "Value 1",
+    "string2": "Value 2"
+  }
+}

tests/testing_config/custom_components/switch/.translations/test.es.json

@@ -0,0 +1,5 @@
+{
+  "state": {
+    "string1": "Spanish Value 1"
+  }
+}