split english language files to frontend and backend

[CptMeatball]

Still a work in progress.

This PR splits the current language file into two seperate file. One for the frontend (lang/xx/frontend.php) and one for the backend (lang/xx/lang.php). This prevents the need of translating the entire file when only front-end translations are needed. It hopefully makes it easier for other contributors to add their own language.

What needs to be done:

• [x] Replace CookieBanner language strings (::lang. with ::frontend.) Done, awaiting check
• [x] Replace CookieManager language strings (::lang. with ::frontend.) Done, awaiting check
• [x] Replace ConsentManager language strings (::lang. with ::frontend.) Are there any?
• [x] Remove unused strings in old language file.
• [ ] Update existing translations to new format
• [ ] Create contributing guidelines for translating files (optional)

How it works

On the frontend the current language strings are replaced with updated strings that point to the new frontend language file.

e.g.

// Old syntax
{{ 'offline.gdpr::lang.commons.links' | trans }}
// New syntax
{{ 'offline.gdpr::frontend.commons.links' | trans }}

(PR for issue #50 )

[CptMeatball]

@tobias-kuendig Can you check if I've covered all language strings in the components? Two pair of eyes see more than one.

[tobias-kuendig]

Looking great so far! I guess the cookie_manager.description string is only used in the backend so this can be removed.