Document Translations Setup + Remove Unused Strings (#2329)

- [x] Remove documentation on how to add discouraged Bower dependencies.
      **TODO:** Document how to pull in npm dependencies.
- [x] Document Transifex setup for pulling translations.
- [x] Remove unused translation strings.

CONTRIBUTING.md

@@ -258,37 +258,14 @@ yarn build-release
 
 Then, run the tests using `grunt test-release:osx --dir=release`, replacing `osx` with `linux` or `win` depending on your platform.
 
-## Dependencies
+## Translations
 
-**Note**: You probably won't end up doing this. Feel free to skip for now.
+To pull the latest translations, follow these steps:
 
-Dependencies are managed by [bower](http://bower.io) and built with
-[grunt](http://gruntjs.com). To change them, you'll need to install node and
-npm, then run `npm install` to install bower, grunt, and related plugins.
-
-### Adding a bower component
-
-Add the package name and version to bower.json under 'dependencies' or `bower install package-name --save`
-
-Next update the "preen" config in bower.json with the list of files we will
-actually use from the new package, e.g.:
-
-```
-  "preen": {
-    "package-name": [
-      "path/to/main.js",
-      "directory/**/*.js"
-    ],
-    ...
-  }
-```
-
-If you'd like to add the new dependency to js/components.js to be included on
-all html pages, simply append the package name to the concat.app list in
-`bower.json`. Take care to insert it in the order you would like it
-concatenated.
-
-Now, run `grunt` to delete unused package files and build `js/components.js`.
-
-Finally, stage and commit changes to bower.json, `js/components.js`,
-and `components/`. The latter should be limited to files we actually use.
+1.  Download Transifex client:
+    https://docs.transifex.com/client/installing-the-client
+2.  Create Transifex account: https://transifex.com
+3.  Generate API token: https://www.transifex.com/user/settings/api/
+4.  Create `~/.transifexrc` configuration:
+    https://docs.transifex.com/client/client-configuration#-transifexrc
+5.  Run `yarn grunt tx`.

_locales/en/messages.json

@@ -338,10 +338,6 @@
       "message": "You don’t have any documents in this conversation",
       "description": "Message shown to user in the media gallery when there are no messages with document attachments (anything other than images or video)"
     },
-    "messageCaption": {
-      "message": "Message caption",
-      "description": "Prefix of attachment alt tags in the media gallery"
-    },
     "today": {
       "message": "Today",
       "description": "Section header in the media gallery"

js/views/message_detail_view.js

@@ -165,7 +165,6 @@
           received: i18n('received'),
           errorLabel: i18n('error'),
           deleteLabel: i18n('deleteMessage'),
-          retryDescription: i18n('retryDescription'),
         })
       );
       this.view.$el.prependTo(this.$('.message-container'));