Rework env-var and project-modules tables to use ff-data-table

### Description

We have a couple of table views that do not use the ff-data-table component - env vars and project modules.

There were some specific UX features those tables needed that weren't immediately obvious on how to achieve with ff-data-table - but on reflection, it should be doable.

Specifically, unlike many uses of ff-data-table we already have, these are _editable_ tables. Rather than just presenting data, they need to be able to edit cells (under certain conditions) and to add/remove rows.

See https://github.com/flowforge/flowforge/pull/1090#issuecomment-1282003374 for context on this.

This task is to rework those two views to use the common component - and to ensure a more consistent UX between them.

### Epic/Story

_No response_