add extra_cookie_attrs option to config

Allow instance admins to set their own SameSite cookie policy from
the config. Default value in the config is `Lax`.

config/config.exs

@@ -154,7 +154,10 @@ config :pleroma, Pleroma.Web.Endpoint,
   signing_salt: "CqaoopA2",
   render_errors: [view: Pleroma.Web.ErrorView, accepts: ~w(json)],
   pubsub: [name: Pleroma.PubSub, adapter: Phoenix.PubSub.PG2],
-  secure_cookie_flag: true
+  secure_cookie_flag: true,
+  extra_cookie_attrs: [
+    "SameSite=Lax"
+  ]
 
 # Configures Elixir's Logger
 config :logger, :console,

docs/config.md

@@ -221,6 +221,8 @@ This section is used to configure Pleroma-FE, unless ``:managed_config`` in ``:i
   - `scheme` - e.g `http`, `https`
   - `port`
   - `path`
+* `extra_cookie_attrs` - a list of `Key=Value` strings to be added as non-standard cookie attributes. Defaults to `["SameSite=Lax"]`. See the [SameSite article](https://www.owasp.org/index.php/SameSite) on OWASP for more info.
+
 
 
 **Important note**: if you modify anything inside these lists, default `config.exs` values will be overwritten, which may result in breakage, to make sure this does not happen please copy the default value for the list from `config.exs` and modify/add only what you need
@@ -442,6 +444,8 @@ The server should also be started with `OAUTH_CONSUMER_STRATEGIES="..." mix phx.
 
 Note: each strategy requires separate setup (on external provider side and Pleroma side). Below are the guidelines on setting up most popular strategies.  
 
+Note: make sure that `"SameSite=Lax"` is set in `extra_cookie_attrs` when you have this feature enabled. OAuth consumer mode will not work with `"SameSite=Strict"`
+
 * For Twitter, [register an app](https://developer.twitter.com/en/apps), configure callback URL to https://<your_host>/oauth/twitter/callback
 
 * For Facebook, [register an app](https://developers.facebook.com/apps), configure callback URL to https://<your_host>/oauth/facebook/callback, enable Facebook Login service at https://developers.facebook.com/apps/<app_id>/fb-login/settings/