Bazel: Remove usage of managed_directories

Bazel removed `managed_directories` feature on Bazel@HEAD, see: [1].
Design document is here: [2].

Also extend the .bazelignore and add explicitly the path to the
node_modules directories. Note, that Bazel currently doesn't support
the same semantic as .gitignore. For more details see this issue: [3]
and this issue specific to rules_nodejs: [4].

[1] bazelbuild/bazel#15463
[2] https://docs.google.com/document/d/1u9V5RUc7i6Urh8gGfnSurxpWA7JMRtwCi1Pr5BHeE44/edit
[3] bazelbuild/bazel#7093
[4] bazelbuild/bazel#8106

Release-Notes: skip
Change-Id: I5dc582e05e4116064fc06d438d4b8a8b57b6bb8d

.bazelignore

@@ -1,2 +1,6 @@
 eclipse-out
 node_modules
+polygerrit-ui/app/node_modules
+polygerrit-ui/node_modules
+plugins/node_modules
+tools/node_tools/node_modules

WORKSPACE

@@ -17,13 +17,6 @@
 #    usages of code from devDependencies in polygerrit bundle.
 workspace(
     name = "gerrit",
-    managed_directories = {
-        "@npm": ["node_modules"],
-        "@ui_npm": ["polygerrit-ui/app/node_modules"],
-        "@ui_dev_npm": ["polygerrit-ui/node_modules"],
-        "@tools_npm": ["tools/node_tools/node_modules"],
-        "@plugins_npm": ["plugins/node_modules"],
-    },
 )
 
 load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")