Remove usage of "whitelist", use better terms instead

The attached patch updates to use the key scheme_allow_list and provides a database update for existing sites. Confirmation that the update does what it should:

Before

scheme_whitelist:
  - http
  - https
  - files
  - internal
protocol_style: full
local_paths: ''

Demonstration of successful execution

 ------------ ----------- --------------- ---------------------------------- 
  Module       Update ID   Type            Description                       
 ------------ ----------- --------------- ---------------------------------- 
  pathologic   8101        hook_update_n   8101 - Move existing setting for  
                                           allowed schema to new name        
                                           (#3216612).                       
 ------------ ----------- --------------- ---------------------------------- 
 
  Do you wish to run the specified pending updates? (yes/no) [yes]:
 > yes

>  [notice] Update started: pathologic_update_8101
>  [notice] Update completed: pathologic_update_8101
 [success] Finished performing updates.

After

scheme_allow_list:
  - http
  - https
  - files
  - internal
protocol_style: full
local_paths: ''

Fantastic, thanks! I moved this from a hook_update_N() (which is only really needed when changing the DB schema) to a hook_post_update(). Among other benefits, that avoids the yucky problem of picking a good value of N for this. 😅 Tested locally and confirmed the post_update is working. Tests are passing locally (although there's no test coverage of this setting). Committed to 2.0.x, cherry-picked to 8.x-1.x and pushed.

Onwards!
-Derek

Automatically closed - issue fixed for 2 weeks with no activity.