basic wiki functionality

[FND]

still missing:

• [x] "edit this page" / "create this page" links
• [x] wiki links (Markdown extension)
• [ ] search functionality (global and per-space)
• [ ] recent changes overview (global, per-space and per-page) → #3
• [ ] revision diffs
• [x] large text box in edit mode (perhaps https://github.com/usablica/widearea?)
• [ ] navigation links (e.g. dashboard, space index)

[FND]

I've created a mock-up to explore how different resources are connected and how this affects the nav menu

(that mock-up also provides a new layout, but that's secondary for now - mostly because I haven't started styling contents yet)

feedback from @pads and @cdent welcome