Try to fix #277 by deleting verbose info

Problem
====

Too slow response from stackage.org/feed/.
So slow that my favorite RSS client (Slack's RSS integration) doesn't work due to timeout.
See https://github.com/fpco/stackage-server/issues/277 for details.

How?
====

Delete the content of the feed if stackage.org/feed is given `withDiff=False` as its query parameter.

Why?
====

I can't confirm it's the true cause of the slowdown (because the server is too hard to run on my machine!).
But anyway I think the html content of the feed is too much:
I just want to know the new LTS Haskell is released by the feed.
I'll click the link if I do want to see the detailed updates.

In addition, there's a reason generating the content causes the slowdown:
Other pages using `getSnapshots` (e.g. https://www.stackage.org/snapshots, https://www.stackage.org/)
are not as slow as https://www.stackage.org/feed/.
So the `getSnapshots` query dosen't seem to be the biggest cause.
And the left possible cause is `mkFeed`.

NOTE
====

I've tested nothing because it's too hard to run this app
without configuring my AWS account.

src/Handler/Feed.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE NoImplicitPrelude #-}
 {-# LANGUAGE QuasiQuotes #-}
 module Handler.Feed
@@ -7,10 +8,11 @@ module Handler.Feed
 
 import Data.These
 import Import
+import RIO.Time (getCurrentTime)
 import Stackage.Database
 import Stackage.Snapshot.Diff
 import Text.Blaze (text)
-import RIO.Time (getCurrentTime)
+import Yesod.Core.Handler (lookupGetParam)
 
 getFeedR :: Handler TypedContent
 getFeedR = track "Handler.Feed.getBranchFeedR" $ getBranchFeed Nothing
@@ -25,7 +27,11 @@ mkFeed :: Maybe SnapshotBranch -> [Entity Snapshot] -> Handler TypedContent
 mkFeed _ [] = notFound
 mkFeed mBranch snaps = do
     entries <- forM snaps $ \(Entity snapid snap) -> do
-        content <- getContent snapid snap
+        showsDiff <- doesShowDiff
+        content <-
+          if showsDiff
+            then getContent snapid snap
+            else return mempty
         return FeedEntry
             { feedEntryLink = SnapshotR (snapshotName snap) StackageHomeR
             , feedEntryUpdated = UTCTime (snapshotCreated snap) 0
@@ -54,6 +60,14 @@ mkFeed mBranch snaps = do
     branchTitle (LtsMajorBranch x) = "LTS-" <> tshow x
     title = "Recent Stackage " <> maybe "" branchTitle mBranch <> " snapshots"
 
+    doesShowDiff =
+        (fmap fromPathPiece <$> lookupGetParam "withDiff") >>= \case
+            Just (Just False) -> return False
+            Just (Just True) -> return True
+            Just Nothing -> notFound
+            Nothing -> return True
+
+
 getContent :: SnapshotId -> Snapshot -> Handler Html
 getContent sid2 snap = do
     mprev <- snapshotBefore $ snapshotName snap