Add GraphQL mutations for merge request saved views

Phil Hughesrequested to merge
ph/mrSavedViewsMutations into master

What does this MR do and why?

Adds mergeRequestSavedViewCreate, mergeRequestSavedViewUpdate and mergeRequestSavedViewDelete, along with MergeRequestSavedViewFilterInput and a negated version of it for the not block.

This is the last of four backend merge requests for merge request dashboard saved views:

  1. !247083 (merged) - table, model, JSON schema, factory
  2. !247090 (merged) - services, finder, and authorization policy
  3. !247106 (merged) - GraphQL read API
  4. This merge request - GraphQL mutations

Example mutation:

mutation {
  mergeRequestSavedViewCreate(input: {
    name: "Needs my review"
    filters: {
      state: opened
      reviewerUsername: "root"
      not: { authorUsername: "root" }
    }
  }) {
    errors
    savedView {
      id
      name
      filters
    }
  }
}

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