Add -default-view to choose timeline or grid as the site root

The view named by -default-view (timeline, the default, or grid) is published
at the site root; the other is served under /timeline/ or /grid/. viewBase and
pageURL derive every page path, pager link and the Timeline<->Grid cross-link
from the single setting, and Build removes both timeline/ and grid/ on each run
so flipping the default never leaves a stale root behind.

Reimplemented on trunk (which has albums and the /manage feature) rather than
cherry-picked, since the original was written against an earlier base. Credit
for the edit/delete and default-view ideas goes to Santhosh Raju, now noted in
a README Contributors section.

- main.go: -default-view flag, validated (fatal on anything but timeline/grid)
- module.nix: services.photogallery.defaultView enum option
- templates: timeline/grid/day/album/albums cross-links use TimelineURL/GridURL
- tests: timeline-root, grid-root, and switch-cleanup

manage_test.go

@@ -29,7 +29,7 @@ func newTestServer(t *testing.T) (*Server, string, string) {
 	}
 	writePNG(t, filepath.Join(content, "bar.png")) // no sidecar
 
-	g := NewGenerator(content, output, "http://x", "A", "@a", "bio", "", "f", "s", "m", 60)
+	g := NewGenerator(content, output, "http://x", "A", "@a", "bio", "", "f", "s", "m", 60, viewTimeline)
 	if err := g.Build(); err != nil {
 		t.Fatalf("initial build: %v", err)
 	}