Create a plan for snippets table migration

Problem

Table snippets includes records for both PersonalSnippets (by author_id) and ProjectSnippets (by project_id).

Unfortunately, this structure doesn't support Cells.

• ProjectSnippets should have cell-local scope
• PersonalSnippets should have clusterwide scope

Proposal

1. Create a step by step plan to migrate snippets table
2. Verify the plan with Cells and database team.