[Frontend] Multiple blocking merge request approval rules

Frontend Issue for: https://gitlab.com/gitlab-org/gitlab-ee/issues/1979

Please refer to the description of the parent issue for up-to-date information.

This issue was created to track Frontend weight independently of Backend.

Some details

• Settings form
• MR new / edit form
• MR Widget

Basic initial wireframe here (not final): : https://gitlab.com/gitlab-org/gitlab-ee/uploads/d0d81219351363f0aaf01a498d7e6150/approvals.png

Iteration Plan

• Move MR approval settings. Same behavior, just moved to it's own settings section and refactored into Vue. Here's the design, except we'll keep the existing behavior instead of the blue box:
  
• Update MR approval section in MR widget. Here's the design:
  
• FE for "Multiple blocking merge request approval rules". This is the full implementation (see designs).

FE/BE Plan

Here's a rough draft of the things the FE will need from an API.

(Warning: This if just a rough draft. The routes and property names may not be accurate.)

#############
# 1. Routes #
#############

https://docs.google.com/spreadsheets/d/1LoosFEFsWPmZhB5f6MnGq0fxp1zCoqyGOOG0uFmd2MA/edit?usp=sharing

##############
# 2. Schemas #  
##############

These schemas are referenced by other schemas and the routes above. 
Please see the routes above for context.

ProjectApprovalRuleSettings {
  id:            Id,
  name:          String,     # Name of the approval rule
  users:         [User],     
  groups:        [Groups],   
  approvalsRequired: Integer,    # Number of required approvals for this rule
}

ProjectApprovalRule {
  id:            Id,
  name:          String,     # Name of the approval rule
  approvers:     [User],   # List of elligible approvers for this rule
  approvalsRequired: Integer,    # Number of required approvals for this rule
}

MergeRequestApprovalRuleAttributes {
  id:            Id,      # ID of the ProjectApprovalRule
  approvalsRequired: Integer, # Required count that was set in this MR
}

MergeRequestApprovalRule {
  id:              Id, 
  name:            String,   # Name given to the approval rule
  approvers:       [User],   # List of eligible approvers for this rule
  approvalsRequired:   Integer,  # Number of required approvals for this rule
  approvedBy:      [User],   # List of users that have approved and match this rule  
}

MergeRequestApprovalSummary {
  approvalsLeft:      Integer,     # The number of approvals left
  approvalRulesLeft:  [String],    # The names of the approval rules that are not yet fully approved
  approvedBy:         [User],      # The approvers of this MR
  suggestedApprovers: [User],     
}

MergeRequestApprovalRule {
  [MergeRequestApprovalRule]      # Array of all rules
}

MergeRequest {
  ...,                                              # Existing MR properties
  approvalRulesAttributes: [MergeRequestApprovalRuleAttributes]  # Array of rule ID and overridden counts
}

Notes:

• We'll want the POST and DELETE of project/merge_request/{id}/approvals to also accept the ?type=full parameter.

TODO

• Update approval promotion text _promote_mr_features.html.haml