Add show/hide secret button to applications view

In #332844 (closed) we stopped showing the application secret in plain text on the screen. Now we just have a copy button. A UX improvement could be to add a show/hide button that allows the user to show and manually copy the secret.

Proposal

Screen_Recording_2021-08-10_at_12.42.27_PM

Implementation plan

1. Initialize a Vue application at app/views/shared/doorkeeper/applications/_show.html.haml#L18
2. Use app/assets/javascripts/vue_shared/components/form/input_copy_toggle_visibility.vue to render the application secret.