Prevent users from assigning an invalid issue type to new issues

Prior this change, when creating a new issue, the passed parameter
`issue_type` wasn't validated for its presence. We only checked if the
corresponding policy is allowed.

For example:

When passing issue_type=incident we check if `create_incident` policy
exists.

When passing issue_type=foo the policy check failed because there was no
`create_foo` policy.

When passing issue_type=project, however, passes the policy check for
`create_project` but the following assignment of issue_type in Issue
fails with 500 error because there is no such issue type (WorkItem::Type
soon) "project".

This commit checks the presence of the passed issue_type before checking
the corresponding policy to prevent such error.

app/services/issues/base_service.rb

@@ -84,7 +84,8 @@ module Issues
 
     # @param object [Issue, Project]
     def issue_type_allowed?(object)
-      can?(current_user, :"create_#{params[:issue_type]}", object)
+      WorkItem::Type.base_types.key?(params[:issue_type]) &&
+        can?(current_user, :"create_#{params[:issue_type]}", object)
     end
 
     # @param issue [Issue]

spec/services/issues/build_service_spec.rb

@@ -183,8 +183,8 @@ RSpec.describe Issues::BuildService do
           expect(issue).to be_incident
         end
 
-        it 'cannot set invalid type' do
-          issue = build_issue(issue_type: 'invalid type')
+        it 'cannot set invalid issue type' do
+          issue = build_issue(issue_type: 'project')
 
           expect(issue).to be_issue
         end