Change "Add to Persistent Storage" into "Install Every Time"

This is not a clear finding from the user testing of the Additional Software beta but more a question that matured in my mind.

Why don’t we have:

Add mumble to your additional software?

To install it automatically from your persistent storage when you start Tails.

[Install Only Once] [Install Every Time]

I think that the reasons why we put “Add to Persistent Storage” when doing the paper prototyping where:

• To help people understand the relationship between additional software and the persistent storage.
• To be a bit more consistent with the other notification when there is no persistent storage yet (“Configure Persistent Storage”).

I’m not so sure anymore about these reasons:

• While writing the documentation we found it hard to explain that even when you choose “Install Only Once”, the packages are saved in persistent storage which is contrary what the button suggests when put in opposition to “Add to Persistent Storage”.
• Put out of context, “Install Only Once” vs “Install Every Time” looks like a simplier alternative to choose from.

Feature Branch: feature/15585-install-every-time

Parent Task: #15567 (closed)

Original created by @sajolida on 15585 (Redmine)