add i18n to frontend and public interface footers

[nebulon42]

Description

Footers of public and frontend interface were not ready for i18n. I have changed that and added three new translation keys each to the en.yml files of frontend and public components. They live under footer. This was inspired by the already existing header in the frontend component.
I'm unsure if relevant keys need to be added to other languages as well. If so, please point this out.
I'm unsure why I had to use the I18n.t().html_safe in the public footer. This was not necessary in the frontend footer. If this is a problem, please tell me.

Related JIRA Ticket or GitHub Issue

Fixes #2583

How Has This Been Tested?

Visual testing by running the frontend and public interfaces locally.
Could not run automated tests locally because of You have already activated nokogiri 1.12.4, but your Gemfile requires nokogiri 1.12.3. Not sure what I'm doing wrong.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have authority to submit this code.
• I have added tests to cover my changes.
• All new and existing tests passed.