GitLab

Allow creating Personal Access Tokens through the website

Related to #2979 

- Allow a user to create personal access tokens, and use them to authenticate
- Refactor `API::Helpers` into `API::Helpers::Core` and `API::Helpers::Authentication`

# Tasks

- [ ]  #2979 (!3749)  - Personal Access Tokens
    - [x]  Basic Implementation
        - [x]  Add UI to add "Personal Access Tokens"
        - [x]  Reload `lib/api` on every request
        - [x]  Respect these tokens for API requests
        - [x]  Just a param or a header too?
        - [x]  Allow revoking tokens
        - [x]  Expire tokens
        - [x]  Left bar should have a "PAT" icon
        - [x]  Scopes?
        - [x]  Copy to Clipboard
        - [x]  Show active/inactive tokens separately
            - [x]  No need to check for expired/revoked in the appropriate places
        - [x]  Why does regular ApplicationController check for private token?
        - [x]  Supp...