Pagination view helpers use Rails i18n instead of gettext

The following discussion from !21232 (merged) should be addressed:

• @toupeira started a discussion:

  @manojmj something unrelated to this MR that I noticed in the screenshot: The button says Prev rather than Previous, and looking at the views in app/views/kaminari/gitlab I saw we're still using Rails I18n rather than Gettext. Should we create a follow-up issue to address that?