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

module.nix

@@ -82,6 +82,12 @@ in {
       description = "Photos per page on the timeline and grid views. The timeline packs whole days, so pages vary slightly around this.";
     };
 
+    defaultView = lib.mkOption {
+      type        = lib.types.enum [ "timeline" "grid" ];
+      default     = "timeline";
+      description = "View published at the site root; the other is published under /timeline/ or /grid/.";
+    };
+
     htpasswdFile = lib.mkOption {
       type        = lib.types.nullOr lib.types.path;
       default     = null;
@@ -156,6 +162,7 @@ in {
             "--serif-family"  cfg.serifFamily
             "--ml-family"     cfg.mlFamily
             "--page-size"     (toString cfg.pageSize)
+            "--default-view"  cfg.defaultView
           ] ++ lib.optionals (cfg.htpasswdFile != null) [
             # %d is systemd's credentials directory; LoadCredential below drops
             # the htpasswd file there, readable only by this service.