refactor the naming within plugin new generator

Having a method called `full?`, which checks on :full and :mountable
is very confusing. I renamed `full?` to `engine?` and created a `full?`
method that only checks the `:full` option
unused scope
Merge pull request #8677 from amatsuda/unused_scope_in_test_models

unused scope in AR test model
Address a failure test_remove_column_with_multi_column_index with Oracle database

Not only PostgreSQL, Oracle database adapter drops the multi-column index
if any of the indexed columns dropped by remove_column.
Merge pull request #8678 from yahonda/remove_column_oracle

Address a failure test_remove_column_with_multi_column_index with Oracle
Standardize the use of current_adapter?
remove meaningless AS::FrozenObjectError
Merge pull request #8682 from amatsuda/rm_as_frozen_object_error

remove meaningless AS::FrozenObjectError
Add missing changelog entry for #8676 [ci skip]
Merge pull request #8686 from mekishizufu/changelog_entry

Add missing changelog entry for #8676 [ci skip]
Make content_tag_for work without block

This is version of #8640 for master
No need to pass a empty block to content_tag_for anymore
allow toggling dumps instead of just showing
add source line padding
Merge pull request #8689 from goshakkk/error-page

Add source code left padding on error pages
refactoring to_sym of Symbol in preload
Cleanup some unnecessary CSS on the new error page and reformat some lines.
Merge pull request #8691 from acapilleri/preload_association

refactoring to_sym of Symbol in preload
replace #flatten with Array()
Merge pull request #8692 from lucasmazza/error-page-css

Cleanup the CSS on the new error page.