Add sortMembersBy

[nf-s]

Add sortMembersBy to GroupTraits

Fixes #5081

Replaced #4809

Related to #3123

Add sortMembersBy to GroupTraits. This can be set to sort group member models - For example sortMembersBy = "name" will alphabetically sort members by name.

Example

http://ci.terria.io/sort-members-by/#clean&https://gist.githubusercontent.com/nf-s/f55f14d31f80829a968b4c323a462cf1/raw/9acc82eb82c12214336d47b764c16e5b3fbb48f7/sort-members-by.json

{
  "catalog": [
    {
      "type": "wms-group",
      "name": "No sort",
      "url": "proxy/_0d/http://ereeftds.bom.gov.au/ereefs/tds/wms/ereefs/mwq_gridAgg_P1M"
    },
    {
      "type": "wms-group",
      "name": "Sort by name",
      "url": "proxy/_0d/http://ereeftds.bom.gov.au/ereefs/tds/wms/ereefs/mwq_gridAgg_P1M",
      "sortMembersBy": "name"
    },
    {
      "type": "wms-group",
      "name": "Sort by WMS layers",
      "url": "proxy/_0d/http://ereeftds.bom.gov.au/ereefs/tds/wms/ereefs/mwq_gridAgg_P1M",
      "sortMembersBy": "layers"
    }
  ]
}

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.