wiki: pages in subfolders #272

Wiki pages can only live at the top of .gitbay/wiki/.

• wiki list and hasWiki read one tree level (internal/control/wiki.go wikiPages, internal/httpd/wiki.go hasWiki).
• The web route /{owner}/{repo}/wiki/{page} matches one segment.
• Relative links and images resolve from the wiki root, not the page's folder.
• The sidebar lists names flat.
• wikiRaw serves every file as application/octet-stream, so SVG images in the wiki do not render.

Changes:

• Recursive page listing; a nested page is named by its path, e.g. Architecture/Identity.
• {page...} route.
• Links on a nested page resolve from the page's folder first, then the wiki root, so [[Admin]] still reaches a root page and root pages behave as before.
• Sidebar grouped by folder.
• wikiRaw uses the image content types the repository raw route uses.
• Users and Parity wiki pages updated.

Unblocks moving the architecture package (!474) into the wiki.