Email subscription improvements

[Oaphi]

closes #1351

With this change applied:

• we no longer redirect to 404 when directly accessing /subscriptions/new;
• filled in subscription fields are preserved between submission attempts;
• it becomes possible to change subscription type / qualifier on the fly;
• it also makes possible to change qualifiers on the fly with dynamic category, tag, and user selects;
• users with JavaScript disabled can complete the whole flow;

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.27%. Comparing base (35b5554) to head (7e21641).
⚠️ Report is 46 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	69.86% <100.00%> (+0.10%)	⬆️
helpers	79.20% <100.00%> (+0.18%)	⬆️
jobs	48.57% <ø> (ø)
models	86.12% <100.00%> (+0.20%)	⬆️

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[cellio]

As for qualifier, for tags it's prefilled (good) and for posts it's irrelevant but can be edited. It doesn't look like editing it does anything

I did at least try to warn users in the field's description - do you think it's not noticeable enough? Maybe we should reword it until we are able to provide JS-based reactivity?

How about: "tag name for tag subscriptions, user number for user subscriptions, otherwise leave blank"? (The main point we want to convey is that if there's nothing here already, don't touch it, but that would be a little blunt.)

Also, I forgot to test user subscriptions before; my comment about the tag name being in the first field applies for user names too.

[Oaphi]

How about: "tag name for tag subscriptions, user number for user subscriptions, otherwise leave blank"? (The main point we want to convey is that if there's nothing here already, don't touch it, but that would be a little blunt.)

Agreed on something like that (not sure if that's going to be needed in the first place with the latest updates), except for the "user number", I'd much rather we refer to ids as "ids"

[cellio]

UI changes look good!

I see that category subscriptions are on this list, but I don't think we have a UI affordance for them. I think maybe we did a long, long time ago before we added RSS? Anyone know the status of these (do they work today and need a UI affordance?)?

[Oaphi]

Anyone know the status of these (do they work today and need a UI affordance?)?

I don't think there's any (we should probably add them back) - I haven't been able to find links for categories similar to other types of subscriptions

[Oaphi]

Phew, the updated flow now works like this:

ssr-2025-08-06_20.10.51.mp4

Added fully functioning type-specific selects for qualified subscriptions instead.
Really didn't want this to become another big update, but oh well, should be worth the effort.

[Oaphi]

Users with JavaScript disabled will now always see a simple text field with an explanation to still be able to create subscriptions. I believe this is an acceptable trade-off (we can't initialize searchable selects without JS):