Notifications list view

Problem

Currently, there is no way to see notifications inside of gitlab (note these are not todos). This issue will handle part of the scope of https://gitlab.com/gitlab-org/gitlab-ce/issues/43474

Notion of notification

Something that you have not yet seen/read/know about.

• Will mark automatically as "read" by viewing the detail page of the detail object
• Items will replace each other as newer activity happens on their parent object (issue/mr/epic)
• Only latest 3 notifications on an item will be shown by default, rest is hidden/expandable
• Items will disappear forever after they are marked as "read"

Scope

• New notification list view
• New notifications in top counters