Disallow methods that copy data on large tables

See merge request gitlab-org/gitlab-ce!20021

These are all over 20 GB on GitLab.com. merge_request_diff_commits is several
hundred gigabytes in size.

This notifies developers when calling `find(_by!)` chained on
`execute`. And suggests using the methods from `FinderMethods`. These
will perform the correct authorization checks on the resource when it
is found.

{change_column_type,rename_column}_concurrently both copy data from one column
to another during a migration, which should not be done on GitLab.com. Instead,
we should use background migrations.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

This reverts the addition of the "goldiloader" Gem and all use of it.
While this Gem is very promising it's causing a variety of problems on
GitLab.com due to it eager-loading too much data in places where we
don't expect/can handle this. At least for the time being this means we
have to go back to manually fixing N+1 query problems, but at least
those should not cause a negative impact on availability.

Goldiloader is great, but has several issues with has_many :through relations:

* https://github.com/salsify/goldiloader/issues/12
* https://github.com/salsify/goldiloader/issues/14
* https://github.com/salsify/goldiloader/issues/18

Rather than try to figure out which applies in each case, we should just do the
drudge work of manually disabling autoloading for all relations of this type. We
can always use regular preloading for specific cases, but this way we avoid
generating invalid queries through Goldiloader's magic.