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
Eager loading made to use relation's in_clause_length instead of host's one (fixes #8474)
Test nested fields with AssociationProxy mockup & fix AssociationProxy support in form helper
Add more documentation to TimeWithZone

[ci skip]
fix wrong hash syntax
New hash syntax works only with symbol keys
"SSL everywhere" for GitHub URLs

see: https://github.com/blog/738-sidejack-prevention-phase-2-ssl-everywhere
correct raw GitHub URLs
no more supporting 1.8
a bit of inline code tags
Model.scoped is deprecated in favour of Model.all
scoped_by is deprecated in AR 4
find_all_by is deprecated in AR 4
find_last_by is deprecated in AR 4
find_or_create_by is deprecated in AR 4
find_or_initialize_by is deprecated in AR 4
find + conditions is deprecated in AR 4
Observers was extracted from Active Model as `rails-observers` gem
wrong model name
PUT => PATCH or PUT