Make Emails translatable via transifex

[kobergj]

Email templates are currently stored in https://github.com/owncloud/ocis/tree/master/services/notifications/pkg/email/templates

1. Remove RenderTemplate code. Hardcode subjects and body as constants into a templates.go file.

   • ocis/services/notifications/pkg/service/service.go

     Line 90 in eb2347e

     msg, err := email.RenderEmailTemplate(bodyTemplate, values, s.emailTemplatePath)

   • https://github.com/owncloud/ocis/blob/master/services/notifications/pkg/email/email.go

2. Use Template function to mark subjects and body as translatable strings

   • Compare https://github.com/owncloud/ocis/blob/master/services/userlog/pkg/service/templates.go

3. Add translation steps to notifications service Makefile

   • Compare https://github.com/owncloud/ocis/blob/master/services/userlog/Makefile#L37-L56

4. Not part of the ticket: Actually translate emails. We first need a way to determine the users preferred language.

cc @dragonchaser @mmattel

[micbar]

I found a new extractor which can also extract variables from template files:

https://github.com/vorlif/xspreak#xspreak

[micbar]

@kobergj @2403905 This is a good project for Roman 👍

[kobergj]

The new extractor is better than the one we use now. But we probably need a custom solution anyways because of #5924

[2403905]

@micbar @kobergj
Please suggest:

1. Should I add a new environment variable instead the NOTIFICATIONS_EMAIL_TEMPLATE_PATH?
2. Which one locale should be used until we determine the users preferred language?https://github.com/owncloud/ocis/pull/6038/files#diff-4849177b19f87c6711bf3fb8b7a84991cf06d64c8c8c05ff5e017c9cc26faa22R90