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
A `NullRelation` should represent nothing. Closes #15176.

[Matthew Draper & Yves Senn]