Add resolution mechanism for todos

What does this MR do?

It adds resolved_by_action field which reflects resolution mechanism of given todo item. It will be used for our internal research related to user interactions with todos.

Migration output

➜  gitlab-rails git:(216045-capture-todo-resolution) ✗ be rake db:migrate:down VERSION=20200520103514
WARNING: This version of GitLab depends on gitlab-shell 13.2.0, but you're running Unknown. Please update gitlab-shell.
== 20200520103514 AddTodoResolvedByAction: reverting ==========================
-- remove_column(:todos, :resolved_by_action)
   -> 0.0020s
== 20200520103514 AddTodoResolvedByAction: reverted (0.0113s) =================

➜  gitlab-rails git:(216045-capture-todo-resolution) ✗ be rake db:migrate
WARNING: This version of GitLab depends on gitlab-shell 13.2.0, but you're running Unknown. Please update gitlab-shell.
== 20200520103514 AddTodoResolvedByAction: migrating ==========================
-- add_column(:todos, :resolved_by_action, :integer, {:limit=>2})
   -> 0.0019s
== 20200520103514 AddTodoResolvedByAction: migrated (0.0115s) =================

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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

Closes #216045 (closed)