#748 Consistently make use of the labels in mixins/Translations.vue for visibility settings
The labels for the different visibility/privacy options ('me', 'instance', 'everyone') are gathered in
front/src/mixins/Translations.vue. However, some
.vue files were hardcoding those labels again instead of using the common ones. This MR fixes it by fetching the common mixins labels.
Side effect: sometimes 'Everyone' was used, sometimes 'Everyone, across all instances', which could be confusing. The latter is now used across the whole App.