When no fork network exists for the source projects, we create a new
one with the correct source

The fork network will keep track of the root project as long as it's
present.

Additionally we're delegating missing method calls on GpgKeySubkey to
GpgKey since most of the info required when verifying a signature is
found on GpgKey which is the parent of GpgKeySubkey

Conflicts:
	db/schema.rb

This is used in EE for the storage migration, and we want to use this
in CE as well to be able to migrate projects to hashed_storage.

This is a composite index on (target_project_id, merge_commit_sha, id)
that allows queries such as the following to use a full backwards index
scan:

    SELECT "merge_requests".*
    FROM "merge_requests"
    WHERE "merge_requests"."deleted_at" IS NULL
    AND "merge_requests"."target_project_id" = 13083
    AND "merge_requests"."merge_commit_sha" = 'e80a893f'
    ORDER BY "merge_requests"."id" DESC
    LIMIT 1;

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38507

Migration 20170919211300_remove_temporary_ci_builds_index.rb created a
temporary partial index and tried to drop it at the end of the
migration. In some circumstances apparently it failed to drop the
index and it ended up in our schema.rb.

This accidentally failed to fail due to a bug in the regular
expression for partial indexes which caused the index creation in
schema.rb to be ignored. Now that that's fixed we could be
resurrecting this zombie index from the past in some but not all
databases.

Add a migration to drop this index if it's present to reconcile this
discrepancy.

This reverts commit 9f186db09d7f380cfbde4c0061be77c272518547.