Automatic checks of authentication / instance publicity. Definition of missing OAuth scopes in AdminAPIController. Refactoring.

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+This document contains notes and guidelines for Pleroma developers.
+
+# Authentication & Authorization
+
+## OAuth token-based authentication & authorization
+
+* Pleroma supports hierarchical OAuth scopes, just like Mastodon but with added granularity of admin scopes.
+  For a reference, see [Mastodon OAuth scopes](https://docs.joinmastodon.org/api/oauth-scopes/).
+
+* It is important to either define OAuth scope restrictions or explicitly mark OAuth scope check as skipped, for every 
+    controller action. To define scopes, call `plug(Pleroma.Plugs.OAuthScopesPlug, %{scopes: [...]})`. To explicitly set 
+    OAuth scopes check skipped, call `plug(:skip_plug, Pleroma.Plugs.OAuthScopesPlug <when ...>)`.
+
+* In controllers, `use Pleroma.Web, :controller` will result in `action/2` (see `Pleroma.Web.controller/0` for definition)
+    be called prior to actual controller action, and it'll perform security / privacy checks before passing control to
+    actual controller action. For routes with `:authenticated_api` pipeline, authentication & authorization are expected,
+    thus `OAuthScopesPlug` will be run unless explicitly skipped (also `EnsureAuthenticatedPlug` will be executed
+    immediately before action even if there was an early run to give an early error, since `OAuthScopesPlug` supports
+    `:proceed_unauthenticated` option, and other plugs may support similar options as well). For `:api` pipeline routes,
+    `EnsurePublicOrAuthenticatedPlug` will be called to ensure that the instance is not private or user is authenticated
+    (unless explicitly skipped). Such automated checks help to prevent human errors and result in higher security / privacy
+    for users.
+
+## [HTTP Basic Authentication](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Authorization)
+
+* With HTTP Basic Auth, OAuth scopes check is _not_ performed for any action (since password is provided during the auth,
+    requester is able to obtain a token with full permissions anyways). `Pleroma.Plugs.AuthenticationPlug` and
+    `Pleroma.Plugs.LegacyAuthenticationPlug` both call `Pleroma.Plugs.OAuthScopesPlug.skip_plug(conn)` when password
+    is provided.
+
+## Auth-related configuration, OAuth consumer mode etc.
+
+See `Authentication` section of [`docs/configuration/cheatsheet.md`](docs/configuration/cheatsheet.md#authentication).