Refactor and expose only Gitlab::UrlBuilder.build(record)

```
$ git grep Gitlab::UrlBuilder

app/models/commit.rb:      url: Gitlab::UrlBuilder.build(self),
app/services/issues/base_service.rb:      issue_url = Gitlab::UrlBuilder.build(issue)
app/services/merge_requests/base_service.rb:      hook_data[:object_attributes][:url] = Gitlab::UrlBuilder.build(merge_request)
app/views/search/results/_note.html.haml:- note_url = Gitlab::UrlBuilder.build(note)
lib/gitlab/note_data_builder.rb:        base_data[:object_attributes][:url] = Gitlab::UrlBuilder.build(note)
spec/lib/gitlab/note_data_builder_spec.rb:    expect(data[:object_attributes][:url]).to eq(Gitlab::UrlBuilder.build(note))
spec/lib/gitlab/url_builder_spec.rb:describe Gitlab::UrlBuilder, lib: true do
```

Fixes #15202.

See merge request !3696

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

Allow Omniauth providers to be marked as external

Related to #4009 

With this MR we will be able to allow the user to set which Omniauth Providers they would like to have as external. All users login in via these providers will be marked as external, even if they already had an account before. If the provider is removed form the list of external providers, the users will be marked as internal at their next login.

MR for Omnibus: gitlab-org/omnibus-gitlab!727

/cc @dblessing @DouweM 

See merge request !3657

Add `Gitlab.com?` method

To be used as a feature flag for GitLab.com-only features, such as
welcome emails.

We will be careful to only use this to disable features or functionality
that do not make sense for any installations that aren't GitLab.com. We
will not use this to restrict features from other installations or keep
them "exclusive" to GitLab.com.

See merge request !3682

One use case for this is manually setting the "action" tag for Grape API
calls. Due to Grape running blocks there are no human readable method
names that can be used for the "action" tag, thus we have to set these
manually on a case by case basis.

To be used as a feature flag for GitLab.com-only features, such as
welcome emails.

We will be careful to only use this to disable features or functionality
that do not make sense for any installations that aren't GitLab.com. We
will not use this to restrict features from other installations or keep
them "exclusive" to GitLab.com.

If the measure method uses Transaction.current directly the SQL
subscriber (Subscribers::ActiveRecord) will add timings of queries
triggered by DB cleaner.