feat: create advanced access permissions for pages resource

[Regaez]

Closes #58

Implements a more scalable and customisable authorisation of page resources as described in this comment: https://github.com/Regaez/grav-plugin-api/issues/58#issuecomment-611953547

Tasks

• [x] Refactor AuthMiddleware to use new AuthHelper class
• [x] Create new api.pages_advanced_access role
• [x] Create new TaxonomyHelper class to determine shared taxonomy values
• [x] Enable advanced access for getPage handler
• [x] Enable advanced access for updatePage handler
• [x] Enable advanced access for deletePage handler
• [x] Enable advanced access for getPages handler
• [x] Enable advanced access for createPage handler
• [x] Create a blueprint to extend the user/account and add API advanced access form fields
• [x] Add tests for new functionality

[Regaez]

👍