Move localization text out of Diffs Vuex Actions and into the i18n localization file

!103303 (merged) modified some UI text that is displayed to the user.

I don't think it's correct that Vuex Actions should internally be responsible for localizing UI text.

As a minor improvement, we should move all of the localized text from the actions.js file into the Diffs app's standard i18n.js file.