privacy: adjust design and text of device security page, especially HSI-1 security level

Tweak the text slightly to improve clarity. For example, instead of just saying "Checks Failed" let's say "Security Checks Failed." There is room to make the strings slightly longer.

The hardest part here was deciding what to say about the HSI-1 security level, which was not described very well prior to these changes. I've made the largest changes at this level. Notably, I have an optional separate commit that changes HSI-1 from green to yellow, to much more clearly indicate that it's somewhere between HSI-0 and HSI-2 in goodness/badness. This is separate from the other changes to ensure the other changes can still land if using yellow is too controversial, but I think it's worth doing. Notably, I've chosen to use channel-secure-symbolic rather than channel-insecure-symbolic at this level because it's still considered a secure state and not an insecure state, though it's not as great as HSI-2. The new states look like this:

HSI-0 (after changes):

HSI-1 (after changes) (screenshot updated April 15):

HSI-2 (after changes):

HSI-1 in more detail:

Before	After (screenshot updated April 15)

HSI-1, before changes; note it is confusingly-similar to HSI-2 and above:

CC for review: @doremihsuan @hughsie @aday