Run sanity checks before migration

[ghost]

Sanity checks, all backed by ITs:

• [x] Fail if versions don't match (including when versions cannot be determined)
• [x] Fail if list of tables don't match (including when tables cannot be determined)
• [x] Fail if list of tables don't correspond to expected list according to version, in both source and target
• [x] Fail if target has projects, issues, or unexpected users
• [x] Fail if source doesn't have projects
• [x] Fail if duplicate values are found in projects.kee of the source (print duplicates)

[ghost]

Fixed by #25