Silverstripe 5 support

[elliot-sawyer]

I've started the work to upgrade this module to Silverstripe 5 and have managed it with very few changes required.

• upgrade this module for SS5 support
• upgrade silverstripe/silverstripe-omnipay for SS5 (merged, needs stable tag)
• upgrade silvershop/silverstripe-listsorter (merged, tagged 3.0.0)
• upgrade silvershop/silverstripe-sqlquerylist (merged, needs stable tag)

It's not mergeable as-is unless you add my forks to the repositories key to your parent project (composer seems to ignore the upstream ones). The pull requests need to be accepted, so I can update this pull request to use a stable tag, or a specific commit until it is marked stable

    "repositories": {
        "silvershop/core": {
            "type": "vcs",
            "url": "git@github.com:elliot-sawyer/silvershop-core.git"
        },
        "silvershop/silverstripe-listsorter": {
            "type": "vcs",
            "url": "git@github.com:elliot-sawyer/silverstripe-listsorter.git"
        },
        "silvershop/silverstripe-sqlquerylist": {
            "type": "vcs",
            "url": "git@github.com:elliot-sawyer/silverstripe-sqlquerylist.git"
        }
    },

The only coding change required was on sqlquerylist, which changed the method signature on limit to support type hinting and return types. There may be more to be checked inside the tests, but I haven't gone into those.

Once those upstream dependencies are merged in, this should be safe to merge

[elliot-sawyer]

Unrelated to the above, when I tried to install silvershop/silverstripe-hasonefield it results in a 500 error when adding it to a DataObject's CMS fields (I think it was Order). That was in a DataExtension so quite possible I was doing something wrong