Maintenance Request Stage transition

This module adds a simple way to control how maintenance requests move from one stage to the next.

On each maintenance stage you can now fill in two new lists: the next stages and the previous stages. You also pick a button colour (primary, info, success, warning or danger) for that stage; if you leave it empty, the primary colour is used.

The stage list screen shows the button colour and the next stages as tags, so you can review the flow at a glance. A stage cannot list itself as a next stage.

On the maintenance request form, the stage field is no longer clickable. Instead, the header gets one button per stage, labelled "To <stage name>". Each button only appears when the request's current stage lists that stage as a next stage, so users only see the transitions that are allowed. Clicking a button moves the request to that stage.

The module touches maintenance.stage and maintenance.request. Demo data pre-configures the standard stages with example transitions and colours.