Validate user id for users select autcomplete ## What does this MR do? Adds digit validation to user_id when build user autocomplete URL for select2. ## Why was this MR needed? To prevent building invalid URL for non existing users like GET "/autocomplete/users/whatever@example.com.json". As result it fixes failing test: ``` Feature: Admin Groups Scenario: Invite user to a group by e-mail ✔ Given I sign in as an admin # features/steps/shared/authentication.rb:11 ✔ And I have group with projects # features/steps/admin/groups.rb:17 ✔ And User "John Doe" exists # features/steps/shared/user.rb:4 ✔ And I visit admin groups page # features/steps/shared/paths.rb:194 ✔ When I visit admin group page # features/steps/admin/groups.rb:9 ✔ When I select user "johndoe@gitlab.com" from user list as "Reporter" # features/steps/admin/groups.rb:48 ✔ Then I should see "johndoe@gitlab.com" in team list in every project as "Reporter" # features/steps/admin/groups.rb:63 /Users/dzaporozhets/.rvm/gems/ruby-2.3.1/gems/actionpack-4.2.7.1/lib/action_dispatch/middleware/debug_exceptions.rb:21:in `call': No route matches [GET] "/autocomplete/users/johndoe@gitlab.com.json" (ActionController::RoutingError) ``` See merge request !6889