NEW Provide a standardised CMSEditLink method

[GuySartorelli]

Provides a standard implementation and method signature for the CMSEditLink() method which is used in various places. See related PRs.
This change allows us to convert the public CMSEditLink() method on CMSEditLinkExtension to instead be an appropriate protected updateCMSEditLink() extension hook. See silverstripe/silverstripe-admin#1809

If we were doing this from scratch I'd call it getEditLink() - if you want that change, since this is a major release I can do that, but bear in mind that change introduces upgrade pain with probably 0 value added, so don't ask for it unless you can indicate some value it adds to offset the upgrade pain it'd introduce.

Issue

• Make extension hook methods protected - part 2 #11258

[GuySartorelli]

Moved from silverstripe/silverstripe-cms#2987

[emteknetnz]

Could we change it to getEditLink(), rename the extension hook updateEditLink and then retain CMSEditLink() as a method that simply calls getEditLink(), deprecate but don't remove it until CMS 7?

[GuySartorelli]

Discussed in person - decided not to rename it given we have other CMS related methods in DataObject (e.g. getCMSFields()) and both being consistent with that naming scheme and being explicit that the edit link is for editing within the CMS is probably preferable.