Add Turkish translation files

Files added:

```
create mode 100644 app/assets/javascripts/locales/tr_TR.js.erb
create mode 100644 config/locales/client.tr_TR.yml
create mode 100644 config/locales/server.tr_TR.yml
create mode 100644 plugins/poll/config/locales/client.tr_TR.yml
create mode 100644 plugins/poll/config/locales/server.tr_TR.yml
create mode 100644 public/403.tr_TR.html
create mode 100644 public/422.tr_TR.html
create mode 100644 public/500.tr_TR.html
create mode 100644 public/503.tr_TR.html
create mode 100644 vendor/gems/discourse_imgur/lib/discourse_imgur/locale/server.tr_TR.yml
```

cc @nlalonde @coding-horror