Submission-channel checkboxes on the Settings page

[ArendPeter]

Parent

Part of #1603.

What to build

On the election Settings admin page, the owner picks which ballot submission channels are allowed for the election via a checkbox group: Online (browser), Paper ballots (admin upload), and (behind a feature flag) Discord. This is the UI for the allowed_submit_types setting added in #1604.

Acceptance criteria

• One allowed_submit_types control on the Settings page, styled as a FormGroup of three checkboxes — not three independent toggles, since they share one array-valued setting: "Online (browser)", "Paper ballots (admin upload)", "Discord".
• Each checkbox's checked state reads (election.settings.allowed_submit_types ?? DEFAULT_ALLOWED_SUBMIT_TYPES).includes(<that type>); toggling one writes the full updated array back.
• The whole group is editable only while the election is in draft, matching the existing pattern used by every other election-settings toggle.
• "Paper ballots (admin upload)" is unchecked by default (inherited from the DEFAULT_ALLOWED_SUBMIT_TYPES default in Enforce allowed_submit_types in the ballot-submission pipeline #1604 — no separate default logic needed here).
• The last remaining checked box is disabled client-side so it can't be unchecked (the server-side empty-array rejection from Enforce allowed_submit_types in the ballot-submission pipeline #1604 is the backstop, not the primary UX).
• The Discord row is hidden entirely behind a new feature flag, following the existing pattern used for the precinct-grouping feature flag. Discord stays policy-allowed by default in the underlying array regardless of the flag.
• Manual/E2E check: toggling "Paper ballots (admin upload)" on in draft, saving, and confirming the setting persists and round-trips through the Settings page.

Blocked by

#1604 (needs the allowed_submit_types field, default, and validation to exist).

[ArendPeter]

Here's the terminology we decided on

Where can voters cast their ballot?
 - Online (on BetterVoting.com)
 - Paper Ballots (uploaded by admin)
 - Discord