Add Czech language option and locale data (#8594)

* Added Czech language option

* Added Czech language option

* Add Czech locale data

* Delete duplicate locale-data/cs.js

Duplicate locale-data is ignored in Mastodon.

see https://github.com/tootsuite/mastodon/blob/v2.5.0/config/webpack/generateLocalePacks.js#L25-L35

app/helpers/settings_helper.rb

@@ -8,6 +8,7 @@ module SettingsHelper
     bg: 'Български',
     ca: 'Català',
     co: 'Corsu',
+    cs: 'Čeština',
     cy: 'Cymraeg',
     da: 'Dansk',
     de: 'Deutsch',

config/application.rb

@@ -42,6 +42,7 @@ module Mastodon
       :bg,
       :ca,
       :co,
+      :cs,
       :cy,
       :da,
       :de,