fix(users/Profpatsch/openlab-tools): add cache headers

This is a dumb experiment to see how hard it is to respect cache
headers; turns out, medium hard but doable.

Sets the correct expiry time according to the cache, plus respects
`If-Modified-Since` which is a tiny bit harder.

Change-Id: I9e6166af0fa254df2beb0f3919187b91a407487b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9810
Autosubmit: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/openlab-tools/openlab-tools.cabal

@@ -95,7 +95,9 @@ library
         wai,
         warp,
         tagsoup,
-        time
+        time,
+        stm,
+        case-insensitive
 
 executable openlab-tools
     import: common-options