Clarify the UX for Independent Container Updates

[almet]

For #1006, the UX story is not yet fully defined.

Currently, the UX is that you can opt-in so that Dangerzone checks for new updates on your behalf. When a new release is detected, a dot is appended to the hamburger menu, and you're notified that there is an update.

Here is a round of questions:

• Should we do the same thing for independent container updates?
• To which level of detail / complexity do we want to expose our users?
• Should we merge the two "updates" settings together in the UI?

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I have my takes on these, but I'm curious what others think about it, especially @harrislapiroff

[almet]

The current take on the UX is that a single setting will be used "Check for updates and install when possible".

A future version could separate checking and installing though.

Having some way of clarifying that the component being updated will not change the current UI or functionality would make things clearer ("DZ will update the sandbox" or "DZ will apply security patches" or something)

It is also important to note that these are updates that need to be applied for the software to securely operate. We want the language to reflect this is core to the secure functioning of Dangerzone.