fix: Sessions generator adds bcrypt more robustly

Use Thor's `uncomment_lines` instead of `gsub_file`.

If the Gemfile does not contain bcrypt, either commented or
uncommented, then run `bundle add bcrypt`.

Make sure all bundler commands run within `Bundler.with_original_env`
to avoid bundler resolution problems while trying to resolve bundler
dependencies. Typically the failure mode here is seeing:

> Could not find bcrypt-3.1.20 in cached gems or installed locally (Bundler::GemNotFound)

while bootstrapping the process that will run bundle-install or bundle-add.

railties/lib/rails/generators/rails/sessions/sessions_generator.rb

@@ -20,9 +20,12 @@ def configure_application
       end
 
       def enable_bcrypt
-        # FIXME: Make more resilient in case the default comment has been removed
-        gsub_file "Gemfile", /# gem "bcrypt"/, 'gem "bcrypt"'
-        execute_command :bundle, ""
+        if File.read("Gemfile").include?('gem "bcrypt"')
+          uncomment_lines "Gemfile", /gem "bcrypt"/
+          Bundler.with_original_env { execute_command :bundle, "" }
+        else
+          Bundler.with_original_env { execute_command :bundle, "add bcrypt" }
+        end
       end
 
       def add_migrations