Allow `not` filtering in issue/mr/epic list views and boards

This issue has moved to GitLab CE. View the active issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/64213

• Search for issues, merge requests, and epics in the search bar using a not operator.
• Same for search bar in boards
• Currently, all search filters/attributes are implicitly AND-ed together. This feature does not change that. It only adds a not condition to individual filters.
• The not condition applies to labels, authors, assignees, and milestones.
• The not condition also includes "absence":
  • not(assignee: @victorwu) includes unassigned
  • not(label: ~Discussion) includes unlabeled
  • not(milestone: %"9.1") includes no milestone

Example

author: @victorwu assignee: @victorwu assignee: @smcgivern not(assignee: @jschatz1) label: ~Discussion not(milestone: %"9.3")

This means find all issues that:

• Are authored by @victorwu
• AND have @victorwu as an assignee
• AND have @smcgivern as an assignee
• AND do not have @jschatz1 as an assignee
• AND has the label gitlab-ce~874211
• AND does not have milestone gitlab-ce%39

Tier

See parent epic for tier explanation &291