Create a new table for instance_integrations

In order to support integrations in Cells architecture we need to split existing integrations into group/project integrations (existing integrations table) and instance wide integrations. Instance wide integrations need a separate table, which should be implemented as part of this issue.

Proposed solution

Create a new instance_integrations table, which is identical to integrations table, with the exception of group_id & project_id foreign keys. instance value is always set to true.

(
    id                                integer default nextval('integrations_id_seq'::regclass) not null
        primary key,

    created_at                        timestamp,
    updated_at                        timestamp,
    active                            boolean default false                                    not null,
    push_events                       boolean default true,
    issues_events                     boolean default true,
    merge_requests_events             boolean default true,
    tag_push_events                   boolean default true,
    note_events                       boolean default true                                     not null,
    category                          varchar default 'common'::character varying              not null,
    wiki_page_events                  boolean default true,
    pipeline_events                   boolean default false                                    not null,
    confidential_issues_events        boolean default true                                     not null,
    commit_events                     boolean default true                                     not null,
    job_events                        boolean default false                                    not null,
    confidential_note_events          boolean default true,
    deployment_events                 boolean default false                                    not null,
    comment_on_event_enabled          boolean default true                                     not null,
    instance                          boolean default false                                    not null,
    comment_detail                    smallint,
    inherit_from_id                   bigint
        constraint fk_services_inherit_from_id
            references integrations
            on delete cascade,
    alert_events                      boolean,

    type_new                          text
        constraint check_a948a0aa7e
            check (char_length(type_new) <= 255),
    vulnerability_events              boolean default false                                    not null,
    archive_trace_events              boolean default false                                    not null,
    encrypted_properties              bytea,
    encrypted_properties_iv           bytea,
    incident_events                   boolean default false                                    not null,
    group_mention_events              boolean default false                                    not null,
    group_confidential_mention_events boolean default false                                    not null
);