Add new cop Gitlab/Rails/SafeFormat

What does this MR do and why?

Enforce safe_format for externalized strings with interpolations and .html_safe.

# bad
_('string %{open}foo%{close}').html_safe % { open: '<b>'.html_safe, close: '</b>'.html_safe }
format(_('string %{open}foo%{close}').html_safe, open: '<b>'.html_safe, close: '</b>'.

# good
safe_format(_('string %{open}foo%{close}'), tag_pair(tag.b, :open, :close)

# also good no `html_safe
format(_('string %{var} number'), var: var)

Contributes to #374091 (closed).

See https://docs.gitlab.com/ee/development/i18n/externalization.html#html

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