MM-57839: Rewrite static assets only if needed (#27076)

* Rewrite static assets only if needed

Refactor UpdateAssetsSubpathInDir so that the actual rewriting of files
happen in two different functions: one for root.html, another for
manifest.json and *.css files.

I would have wanted to simply do a

  if pathToReplace == newPath { return nil }

when those two variables are defined, but the logic for root.html is not
that simple, and that may miss some edge cases, so I opted out for
simply moving the actual rewriting to individual functions, which check
for their corresponding conditions to perform the update:
  1. for root.html, check that the edited file is indeed different than
     the original one
  2. for manifest.json and *.css files, rewrite them only if
     pathToReplace != newPath, which in this case is clear that that's the
     only modification we do

* Fix expected error msg in test

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/utils/subpath_test.go

@@ -109,7 +109,7 @@ func TestUpdateAssetsSubpath(t *testing.T) {
 				baseCSS,
 				baseManifestJSON,
 				"/subpath",
-				fmt.Errorf("failed to find 'Content-Security-Policy' meta tag to rewrite"),
+				fmt.Errorf("failed to update root.html: %w", fmt.Errorf("failed to find 'Content-Security-Policy' meta tag to rewrite")),
 				contentSecurityPolicyNotFoundHTML,
 				baseCSS,
 				baseManifestJSON,
@@ -120,7 +120,7 @@ func TestUpdateAssetsSubpath(t *testing.T) {
 				baseCSS,
 				baseManifestJSON,
 				"/subpath",
-				fmt.Errorf("failed to find 'Content-Security-Policy' meta tag to rewrite"),
+				fmt.Errorf("failed to update root.html: %w", fmt.Errorf("failed to find 'Content-Security-Policy' meta tag to rewrite")),
 				contentSecurityPolicyNotFound2HTML,
 				baseCSS,
 				baseManifestJSON,