Add notification subscription feature to swimlanes sidebar

What does this MR do?

• The changes are behind this FF:

  • :boards_with_swimlanes

• Adds notification subscription feature to swimlanes sidebar to close #36321 (closed)

  • board_sidebar_subscription.vue is the newly added component in this MR.

• Factors out projectPath computed property and create a new getter projectPathForActiveIssue. There have been duplicates/identical copies of this computed property used in the following places:

  • ee/app/assets/javascripts/boards/components/sidebar/board_sidebar_epic_select.vue
  • ee/app/assets/javascripts/boards/components/sidebar/board_sidebar_weight_input.vue
  • app/assets/javascripts/boards/components/sidebar/board_sidebar_due_date.vue
  • app/assets/javascripts/boards/components/sidebar/board_sidebar_labels_select.vue

Swimlane sidebar is accessible when issues are grouped by epics:

Screenshots (strongly suggested)

Demo
Screen_Recording_2020-11-10_at_2.01.34_PM

when notifications are disabled (emailsDisabled=true)

To set emailsDisabled to true, navigate to the general settings for the project:

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