Update some i18n references in guides

[shioyama]

The Globalize::Backend::Static class no longer exists in the globalize gem, so far as I can tell. I think this section of the guide is way out of date.

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[shioyama]

Summary of changes:

• Remove reference to Globalize::Backend::Static which no longer exists in globalize gem
• Remove reference to rails-i18n google group, which is no longer maintained (last post in 2014, moderators no longer active)
• Remove confusing reference to Globalize3
• Add section on storing translations, reference top three popular gems for doing this (Globalize, Mobility, Traco) (Disclosure: I'm the author of Mobility.)

I think a lot more could be done to update this guide, just skimming it I found these issues but there are probably more. I think adding a section on translating content is important because it's a topic that comes up a lot, and can be very confusing for people unaware of the distinction with the standard I18n API.

Also: I'm aware that technically you can store strings in the database with i18n using the ActiveRecord backend, but it's an edge use case and gems like Globalize and Mobility are much more commonly used for this.

[shioyama]

@svenfuchs Since you were the original author of this guide, maybe you could comment on the changes? 😄

[shioyama]

There were some formatting issues, fixed them in 5cb14f0.

[pixeltrix]

@shioyama thanks!

[shioyama]

Thanks! I don't suppose there's any chance this could get cherry-picked to 5-2-stable as well? Or should I make a PR to that branch for that?

[pixeltrix]

@shioyama cherry-picked to 5-2-stable in 78d47aa

[shioyama]

👍 Thanks!

[svenfuchs]

Thanks for all your great work in the I18n space, @shioyama!

❤️

[shioyama]

😊