Fix all hlint warnings

- Html.hs: replace lambdas with section/composition where possible
- Html.hs: replace case/fromMaybe with fromMaybe
- Html.hs: add fromMaybe to imports, use unqualified
- Parser.hs: replace span with break (2 locations)
- Parser.hs: replace if/then/else with list comprehension
- Parser.hs: remove redundant catch-all patterns (break is exhaustive)
- Server.hs: eta reduce htmlResponse
- Add .hlint.yaml to suppress 'Avoid lambda' (suggested fix has
  precedence issues with blaze-html's infix (!) operator)
- All 26 tests passing, hlint: No hints, exit code 0

src/Roux/Parser.hs

@@ -357,20 +357,18 @@ buildRecipe items =
 groupSections :: [RawBlock] -> [Section]
 groupSections [] = []
 groupSections items =
-    case span (not . isSectionHeader) items of
+    case break isSectionHeader items of
         -- No section header: everything goes into an unnamed section
         (body, []) ->
             [Section Nothing (makeNonEmpty (map rawToBody body))]
         -- Items before the first header → unnamed section, then named
         (before, (RawSectionHeader name) : rest) ->
             let pre =
-                    if null before
-                        then []
-                        else [Section Nothing (makeNonEmpty (map rawToBody before))]
+                    [Section Nothing (makeNonEmpty (map rawToBody before))
+                        | not (null before)
+                    ]
                 named = collectNamedSection name rest
              in pre ++ named
-        -- Unreachable: 'span' never returns a non-header as the first item
-        -- of the second component, but GHC doesn't know that.
         _ -> error "groupSections: impossible"
 
 -- | Is this block a section header?
@@ -381,7 +379,7 @@ isSectionHeader _ = False
 -- | Collect a named section: from the header to the next header (or end).
 collectNamedSection :: Text -> [RawBlock] -> [Section]
 collectNamedSection name items =
-    case span (not . isSectionHeader) items of
+    case break isSectionHeader items of
         (body, []) ->
             [Section (Just name) (makeNonEmpty (map rawToBody body))]
         (body, (RawSectionHeader nextName) : rest) ->