Add member events column to webhooks table

What does this MR do?

First step to implementing #260347 (closed).

This MR just adds a field to the database, UI/API and webhooks themselves will be handled in next MRs.

$ rails db:migrate
== 20201204205814 AddMemberEventsToWebHooks: migrating ========================
-- add_column(:web_hooks, :member_events, :boolean, {:null=>false, :default=>false})
   -> 0.0036s
== 20201204205814 AddMemberEventsToWebHooks: migrated (0.0037s) ===============


$ rails db:rollback
== 20201204205814 AddMemberEventsToWebHooks: reverting ========================
-- remove_column(:web_hooks, :member_events, :boolean, {:null=>false, :default=>false})
   -> 0.0039s
== 20201204205814 AddMemberEventsToWebHooks: reverted (0.0067s) ===============

Screenshots (strongly suggested)

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