models backed by views don't assume "id" columns are the primary key.

Closes #10247.

The same goes for tables with an "id" column but without primary key constraint.
Reading from the view works without configuration. If you have an updateable view
you need to use `self.primary_key = ''`.
Oracle does not support `IF EXISTS` for `DROP VIEW`.

/cc @yahonda
Removed any default margin from body.
Merge pull request #16855 from slavingia/patch-1

Removed any default margin from mail preview body.
[ci skip] application.rb -> development.rb in web-console docs
Merge pull request #16858 from y-yagi/fix_web_console_docs

[ci skip] application.rb -> development.rb in web-console docs
Tell the user which file is missing when config/database.yml was not found

Since cc03675d30b58e28f585720dad14e947a57ff5b the error message became like
"Could not load database configuration. No such file -"
which doesn't really tell what's actually missing.
[ci skip]Correct variables in the sample code
Merge pull request #16861 from suginoy/fix_typo_people_post

[ci skip]Correct variables in the sample code
i18n guide: clarify interpolation rules in _html keys [ci skip]
Replace drop sql statement to drop_table method
to drop sequences at the same time each tables dropped for Oracle
Merge pull request #16867 from yahonda/use_drop_table_to_drop_sequences

Replace drop sql statement to drop_table method