Add Serbian language pack

Thanks to Srđan Šajin

translations/sr@latin/canned_messages.yml

@@ -0,0 +1,2 @@
+- 'Dobar dan! Kako Vam mogu pomoći?'
+- 'Dobar dan! Dobrodošli u našu podršku. Kako Vam mogu pomoći?'

translations/sr@latin/config.yml

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+# This is human-readable name which will be used everywhere in UI.
+name: Srpski
+# Indicates if the locale uses Right-to-Left writing.
+rtl: false
+# Locale name which will be passed into PHP's setlocale function.
+time_locale: sr_RS.UTF8
+# Various formats which will be used with PHP's strftime function.
+date_format:
+    full: "%d.%m.%Y %H:%M"
+    date: "%d.%m.%Y"
+    time: "%H:%M"

translations/sr@latin/mail_templates.yml

@@ -0,0 +1,9 @@
+user_history:
+    subject: 'Mibew Messenger: istorija razgovora'
+    body: "Dobar dan, {0}! \n\nVaša istorija razgovora: \n\n{1} \n--- \nSa poštovanjem, \nMibew Messenger"
+password_recovery:
+    subject: 'Reset your Mibew password'
+    body: "Hi, {0}\n\nPlease click on the link below or copy and paste the URL into your browser:\n{1}\n\nThis will let you choose another password.\n\nRegards,\nMibew"
+leave_message:
+    subject: 'Pitanje od {0}'
+    body: "Imate poruku od {0}: \n\n{2} \n\nNjegov/njezin email: {1} \n{3} \n--- \nVaš Mibew Messenger"