Custom field columns include subprojects fields

Evaluate subprojects of current project to include readable custom
fields in the available columns list.
This allows adding a column for a custom field that is linked to a
subproject, which is the default behaviour when selecting ALL_PROJECTS.
Since columns are related to a project, but the filter by default shows
issues from subproject, extending the behaviour for any
project/subproject is consistent with the expectation that columns for
those custom fields can be used for view-all-bug page

Fixes: 0023233