Change Test Environments from JIRA label field to alternative managed dataset

Currently the Test Environments field leans on the JIRA Label field type. This means that usage of it is prone to typos, misunderstandings, and free input for all test users. With a large test team, this is nearly impossible to manage effectively and leads to many errors in reporting, coverage and requirements statuses.

In addition, due to a JIRA bug, it is currently impossible to make this field read-only - excluding it from the edit screen means it still renders with a "pencil" icon next to it (even though the in-line edit is disabled); clicking this icon allows full access to adding/deleting labels.

https://jira.atlassian.com/browse/JRASERVER-44693

The Test Environments for a team should be fairly fixed and managed, not loose and ad-hoc in a way that Label fields offers - a standard, editable multi select or checkbox field would be far more suited to this (and not subject to the labels bug). It does not need a specific field type.