More privatization, and some error helpers (#2976)

* More privatization, and some error helpers

This makes a lot more things private. Continued from #2967, fixes #2984
Adds public methods for various things.

Below is a brief summary of things available elsewhere, though this
particular set of changes may warrant a detailed section in the release notes.

 - bot.db.locale -> redbot.core.i18n.get_locale
   - Note: This one already existed.
 - bot.db.help -> redbot.core.commands.help.HelpSettings
 - bot db whitelist/blaclist? -> bot.allowed_by_whitelist_blacklist
   - This has also been made a single cannonical function for this
   purpose including check usage
 - bot color? -> bot.get_embed_color/bot.get_embed_colour
 - bot.id.api_tokens? ->

   - bot.get_shared_api_tokens
   - bot.set_shared_api_tokens
   - bot.remove_shared_api_tokens

 -bot.db.prefix -> bot.get_valid_prefixes
   - (Note: This is a wrapper around bot.get_prefix)

 Other changes include
  - removing `bot.counter` as it was never used anywhere
  - Adding properties with helpful error messages for moved and renamed
  things
  - making bot.uptime a property with an error on set
  - adding a migration to the bot config for shared_api_tokens

* Remove overly encompassing message redaction, eval is a risk, dont run in dev if you cant manage it

* address Flame's feedback

* rephrase example

* changelog extras

* You saw nothing

redbot/cogs/downloader/downloader.py

@@ -65,7 +65,7 @@ class Downloader(commands.Cog):
             The default cog install path.
 
         """
-        return await self.bot.cog_mgr.install_path()
+        return await self.bot._cog_mgr.install_path()
 
     async def installed_cogs(self) -> Tuple[Installable]:
         """Get info on installed cogs.