Tokens/Mailings - Split in two files. Multiple cleanups.

Created by: totten

Including the token documentation under CiviMail is misleading -- the docs talk about CiviMail because, historically, much of this grew out of CiviMail. However, today, tokens are used in several more contexts. Also, in civicrm-core, TokenProcessor is not used by CiviMail. (You have to install FlexMailer to use TokenProcessor on mailings... and you may also have to enable an experimental setting...)

Created by: totten

@michaelmcandrew @seamuslee001 - this is following up the conversation on Mattermost about token processing.

Created by: seamuslee001

I looked over this and i am happy with the changes. @michaelmcandrew any thoughts?

Created by: totten

Thanks, @seamuslee001. Will go ahead and merge to prevent staleness - but further feedback from Michael is of course appreciated.