Fix migrations with enable_extension

When using ActiveRecord::Base.table_name_prefix and/or
table_name_suffix, extension names got the same treatment as table names
when running migrations. This led to migrations that tried to call, for
example, enable_extension("prefix_hstore") on the connection.
Allow custom asset host to be passed in asset_url
Improve documentation for asset_url

Updated CHANGELOG.md with entry about :host in asset_url
Fixed duplicate subscribers in ActiveSupport::Subscriber

ActiveSupport::Subscriber no longer creates multiple subscribers when
you redefine a method.
Add RFC4791 MKCALENDAR method
Changed extract_limit in class Column to return correct mysql float and double limits
Merge pull request #14791 from asn62/mysql_double_bug

Changed extract_limit in class Column to return correct mysql float and
double limits
Merge pull request #15121 from skarpesh/rfc4791-methods

Add RFC 4791 MKCALENDAR method
fixes stack level too deep exception on action named 'status' returning 'head :ok'
Merge pull request #15118 from khelben/head_with_status_action_stack_level_too_deep

fixes stack level too deep exception on action named 'status' rendering 'head :ok'
Small refactoring on clear_transaction_record_state

Make sure when we clean the `@_start_transaction_state` var we do it in
the same code-path.
Also this makes `clear_transaction_record_state`, more consistent with `restore_transaction_record_state`
Clear transaction state if callback raise rollback
Fix `Relation#delete_all` inconsistency

When relation scopes include one of `uniq`, `group`, `having` or
`offset`, the generated query ignores them and that causes unintended
records to be deleted. This solves the issue by restricting the deletion
when those scopes are present.

rails/rails#11985
formatting pass through CHANGELOGS. [ci skip]
fixed a typo [ci skip]
Merge pull request #15133 from ShunsukeAida/master

fixed a typo
Document conflicts in `db/schema.rb` [ci skip]
Merge pull request #15113 from leafac/document-schema-conflicts

Document conflicts in `db/schema.rb` [ci skip]
Remove code related to adapter which are no longer in repo

As we have remove code for these adapters so no need to have tasks
Our test suite isn't ready to run in random order yet :cry: