Add backend for system users and groups

[Dave Shoreman]

Certain sites, applications or services may be better ran on their own user.

Frontend/General Checklist

Groups

• List all groups in new System/Groups page
• Selecting a group should open editor in right pane
• Include group users in a list under each group
• List users separately on the edit form with 'Remove from group' option
• Ability to add a user to a group, from Users or Groups page

Users

• Create 'Users' endpoint under new System section
• Display all users (including root and system users) in a list [of cards?]
• Ensure [system-]users can be added via backend+gui
• On edit form, display groups in a dropdown using new API endpoint
• Add a multiselect for the user's secondary groups

Misc

• Add a 'System' area to primary nav

API Checklist

Users

• GET /system/users
• POST /system/users
• PUT /system/users
• DELETE /system/users

Groups

• GET /system/groups
• POST /system/groups
• PUT /system/groups
• DELETE /system/users

Sample 'Create User' API Request

Flags respective to adduser command, useradd may differ.

POST /api/users
Request:
  username: string|required
  group: string|appears_in:groups   // -g
  groups: list[group]               // -G
  is_system: // -r / --system
  has_homedir: bool|default:true    // -m | -M
  homedir: path|default:null        // -d


### Sample Flow for Creating a User
[Enter a username]

<Which type of user is this?> [normal] [system]
<Can this user login?> [yes] [no]
<Set primary group?> [match username]  [select from existing] [custom]

[Dave Shoreman]

Finally finished with the merge of #40!