authz: add a new `synced_at` column to perms tables

Created by: unknwon

The value of the new synced_at column in both user_permissions and repo_permissions tables is used to indicate when was the last user-/repo- centric permissions sync happened.

The existing update_at column isn't sufficient because any incremental permissions update would change its value.

Both values of synced_at and update_at columns will be shown in permissions UI as "Last complete sync" and "Last incremental sync" respectively (these terms were discussed in design sync, unimportant to this PR anyway).

If anyone thinks updated_at is better because of naming consistency, happy to change it too.