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

main.go

@@ -21,15 +21,20 @@ func main() {
 	mlFamily    := flag.String("ml-family", "Manjari", "CSS font-family for the alt-script (Malayalam) bio line")
 	pageSize    := flag.Int("page-size", 60, "photos per page on the timeline and grid views (timeline packs whole days, so pages vary slightly around this)")
 	htpasswd    := flag.String("htpasswd", "", "path to an htpasswd file (bcrypt, `htpasswd -B`) guarding /upload; empty disables auth")
+	defaultView := flag.String("default-view", viewTimeline, "view published at the site root: \"timeline\" or \"grid\"; the other is published under /<name>/")
 	flag.Parse()
 
+	if *defaultView != viewTimeline && *defaultView != viewGrid {
+		log.Fatalf("-default-view must be %q or %q, got %q", viewTimeline, viewGrid, *defaultView)
+	}
+
 	for _, d := range []string{*contentDir, *outputDir} {
 		if err := os.MkdirAll(d, 0755); err != nil {
 			log.Fatalf("mkdir %s: %v", d, err)
 		}
 	}
 
-	g := NewGenerator(*contentDir, *outputDir, *baseURL, *author, *handle, *bio, *bioAlt, *fontsURL, *serifFamily, *mlFamily, *pageSize)
+	g := NewGenerator(*contentDir, *outputDir, *baseURL, *author, *handle, *bio, *bioAlt, *fontsURL, *serifFamily, *mlFamily, *pageSize, *defaultView)
 
 	if err := g.Build(); err != nil {
 		log.Printf("initial build: %v", err)