Minor tweaks to installer text

- use "open wiki" (explicit meaning) rather than "traditional wiki" (implicit meaning)
- tweak sentence that explains what the "traditional wiki" is to work with whatever label is used
- "Traditionally" --> "Usually" (less dogmatic)
- use the system message for the "back" button, to match the "next" button

Change-Id: I92b599965c6ed7e405951a37f2d3cbc8710aff16
Merge "Minor tweaks to installer text"