Reworks snapshots and segments structure

What does this MR do?

Reworks snapshots and segments structure. No user facing changes yet. Those will come in part-3

Previously:

• A snapshot belongs to 1 namespace and has many segments
• A segment belongs to 1 segment

Now:

• A snapshot belongs to 2 namespaces and has many segments
• A segment belongs to 1 namespace

That makes snapshots independent of segments and allows us to create multiple segments with the same snapshots data (no overhead recalculations).

Cleans up obsolete fields on Snapshot and Segment models.

Migration output

== 20210517144856 RequireSnapshotNamespace: reverting =========================
-- execute("ALTER TABLE analytics_devops_adoption_snapshots\nDROP CONSTRAINT IF EXISTS check_3f472de131\n")
   -> 0.0025s
== 20210517144856 RequireSnapshotNamespace: reverted (0.0168s) ================

== 20210517130723 MakeSnapshotSegmentIdOptional: reverting ====================
-- change_column_null(:analytics_devops_adoption_snapshots, :segment_id, false)
   -> 0.0050s
== 20210517130723 MakeSnapshotSegmentIdOptional: reverted (0.0050s) ===========

== 20210517130723 MakeSnapshotSegmentIdOptional: migrating ====================
-- change_column_null(:analytics_devops_adoption_snapshots, :segment_id, true)
   -> 0.0059s
== 20210517130723 MakeSnapshotSegmentIdOptional: migrated (0.0060s) ===========

== 20210517144856 RequireSnapshotNamespace: migrating =========================
-- execute("    UPDATE analytics_devops_adoption_snapshots snapshots\n      SET namespace_id = segments.namespace_id\n    FROM analytics_devops_adoption_segments segments\n    WHERE snapshots.namespace_id IS NULL AND segments.id = snapshots.segment_id\n")
   -> 0.0038s
-- current_schema()
   -> 0.0016s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0021s
-- execute("ALTER TABLE analytics_devops_adoption_snapshots\nADD CONSTRAINT check_3f472de131\nCHECK ( namespace_id IS NOT NULL )\nNOT VALID;\n")
   -> 0.0034s
-- current_schema()
   -> 0.0021s
-- execute("ALTER TABLE analytics_devops_adoption_snapshots VALIDATE CONSTRAINT check_3f472de131;")
   -> 0.0037s
== 20210517144856 RequireSnapshotNamespace: migrated (0.0475s) ================

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #329521 (closed)