refactor(ops/modules): Move cgit configuration into a module

The ancient `//web/cgit-taz` path stems from the time I had
code.tazj.in serving my initial version of the depot.

I've been meaning to clean this up for forever, so here we go.

Note that this leaves the git-serving module in a strange state where
it only deals with josh. I'll rename it accordingly.

Change-Id: I47ed1e9d90958299b5440a18a1b9075274754e33
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5294
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>

ops/machines/whitby/default.nix

@@ -8,6 +8,7 @@ in
 {
   imports = [
     "${depot.path}/ops/modules/atward.nix"
+    "${depot.path}/ops/modules/cgit/default.nix"
     "${depot.path}/ops/modules/clbot.nix"
     "${depot.path}/ops/modules/gerrit-queue.nix"
     "${depot.path}/ops/modules/git-serving.nix"
@@ -392,6 +393,7 @@ in
     nixery.enable = true;
 
     # Run cgit & josh to serve git
+    cgit.enable = true;
     git-serving.enable = true;
 
     # Configure backups to GleSYS