DEV: patchbot: gray the save button when there is nothing to save

DEV: patchbot: gray the save button when there is nothing to save

The "Save changes" button used to remain active all the time, giving no
hint about whether anything was pending. It is now disabled whenever
nothing differs from the reference: no verdict change, no non-empty
note addition, no note edition differing from its base. It gets
re-evaluated after every action which may change that (verdict clicks,
typing in a note input, opening/cancelling a box, updates and saves),
bailing out at the first pending change so the common case stays cheap.

As a side effect, the button lighting up right after a reload confirms
at a glance that the browser restored unsaved local edits.