Create new policies for read, destroy, and create tokens

What does this MR do?

There are a few places in our codebase where can?(:admin_resource_access_tokens) is too broad, for example in the project access token API GET method where the user only needs token read permissions or when group admins want to (WIP) disable the creation of project access tokens but should still be able to read and destroy existing tokens. This MR breaks down the can?(:admin_resource_access_tokens) permission into create_*, read_*, and destroy_*.

It also introduces a column to namespace_settings called resource_access_token_creation_allowed, and introduces a policy called resource_access_token_creation_allowed? to add meaningful specs for the new create_*, read_*, and destroy_* policies, and to facilitate future work on disabling project access tokens.

Migration output:

Up:

% rails db:migrate
== 20210312193532 AddResourceAccessTokenCreationAllowedToNamespaceSettings: migrating 
-- add_column(:namespace_settings, :resource_access_token_creation_allowed, :boolean, {:default=>true, :null=>false})
   -> 0.0030s
== 20210312193532 AddResourceAccessTokenCreationAllowedToNamespaceSettings: migrated (0.0178s) 

Down:

% rails db:rollback STEP=1
== 20210312193532 AddResourceAccessTokenCreationAllowedToNamespaceSettings: reverting 
-- remove_column(:namespace_settings, :resource_access_token_creation_allowed)
   -> 0.0013s
== 20210312193532 AddResourceAccessTokenCreationAllowedToNamespaceSettings: reverted (0.0084s) 

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

app/controllers/projects/settings/access_tokens_controller.rb

@@ -6,7 +6,9 @@ class AccessTokensController < Projects::ApplicationController
       include ProjectsHelper
 
       layout 'project_settings'
-      before_action :check_feature_availability
+      before_action -> { check_permission(:read_resource_access_tokens) }, only: [:index]
+      before_action -> { check_permission(:destroy_resource_access_tokens) }, only: [:revoke]
+      before_action -> { check_permission(:create_resource_access_tokens) }, only: [:create]
 
       feature_category :authentication_and_authorization
 
@@ -43,8 +45,8 @@ def revoke
 
       private
 
-      def check_feature_availability
-        render_404 unless project_access_token_available?(@project)
+      def check_permission(action)
+        render_404 unless can?(current_user, action, @project)
       end
 
       def create_params