Enable RuboCop for migrations

## What does this MR do?

Enable RuboCop for all files inside `db/migrate`, then add magic comments to all existing files, so that this only affects new migrations.

## Are there points in the code the reviewer needs to double check?

This entire change is a config change and a bunch of comments.

## Why was this MR needed?

```
Yorick Peterse [11:55 AM]  
I don't think we have any use case for nested def, might as well blacklist it

Sean McGivern [11:57 AM]  
http://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Lint/NestedMethodDefinition

Sean McGivern [11:57 AM]  
hmm, it's already enabled

Sean McGivern [11:57 AM]  
... because we exclude `db/` from rubocop 🙂

Douwe Maan [11:57 AM]  
@smcgivern: heh

Sean McGivern [11:59 AM]  
I guess that's because we don't want to change the old migrations? I wonder if it's worth enabling it and adding magic comments to all the previous ones to ignore rubocop

Douwe Maan [11:59 AM]  
@smcgivern: agreed
```

## What are the relevant issue numbers?

None.

## Screenshots (if relevant)

None, but if I remove the magic comment from the migration `20160416182152_convert_award_note_to_emoji_award.rb` I get:
```
$ be rubocop
Inspecting 1959 files
..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................W....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Offenses:

db/migrate/20160416182152_convert_award_note_to_emoji_award.rb:3:5: W: Lint/NestedMethodDefinition: Method definitions must not be nested. Use lambda instead.
    def up ...
    ^^^^^^

1959 files inspected, 1 offense detected
```

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- [ ] Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4559