Extract a base class from ActiveSupport::LogSubscriber

Adds a ActiveSupport::Subscriber base class that LogSubscriber inherits
from. By inheriting from Subscriber, other kinds of subscribers can take
advantage of the event attachment system.
Merge pull request #10218 from dasch/dasch/extract-subscriber-class

Extract a Subscriber base class from LogSubscriber
Copy-edits in the ActiveSupport::Subscriber documentation.

Thank you @fxn :green_heart::yellow_heart::heart::purple_heart::blue_heart:

[ci skip]
Ruby On Rails -> Ruby on Rails [ci skip]
Fix #7619. 0x prefix must be added when assigning hexadecimal string into bit column in Postgresql, because solving ambiguity.