Support Multibranch pipelines in the UI

### What feature do you want to see added?

This plugin allows setting job priority in the UI for simple pipelines.

For Multibranch pipelines, the UI will show (read-only) the priority in the UI for individual branches created by the multi-branch pipeline, but the priority field does not appear in the UI for the actual parent multibranch job and it does not seem possible to set the priority on multibranch builds via the UI.

### Upstream changes

_No response_

### Are you interested in contributing this feature?

_No response_

Start by tracing how the existing UI exposes priority for simple pipelines and their individual multibranch branches, then compare that with the parent multibranch job representation. The work is complete when priority can be set for the parent multibranch job and its builds through the UI, with the existing branch display still working.

Written by the indexing model from the issue text.