Add user label settings page

[micahmo]

Pull Request Description

This PR adds a settings page to manage user labels.

• All user labels will be listed (regardless of your current instance).
• A label can be edited by tapping on it, or deleted by tapping the x.
• New labels can be created. First you pick the user (only users on your current instance will be selectable), then you write the label.
  • If you happen to "create" a label that already exists, any changes you make in the dialog will be reflected in the list.

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

qemu-system-x86_64_jxnmwGj9lh.mp4

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

LGTM! Just a couple of very minor points

[hjiangsu]

This is more of a preference thing (so no need to change anything!), but I generally like to return early on these types of boolean conditions to reduce the logic. Of course, this only works if this is the last thing that needs to be performed.

Suggested change

	if (result.deleted) {
	// It exists in our list and was deleted, so remove it.
	setState(() => userLabels.removeWhere((userLabel) => userLabel.username == result.userLabel!.username));
	} else {
	// It exists in our list but was changed, so update it.
	setState(() => userLabels[userLabels.indexOf(existingLabel)] = result.userLabel!);
	}
	if (result.deleted) {
	// It exists in our list and was deleted, so remove it.
	setState(() => userLabels.removeWhere((userLabel) => userLabel.username == result.userLabel!.username));
	return;
	}
	// It exists in our list but was changed, so update it.
	setState(() => userLabels[userLabels.indexOf(existingLabel)] = result.userLabel!);

[micahmo]

That makes sense! In this case, since there's only one line of logic in the else, I think the parallelism of having both statements in conditions helps to see what's going on without too much clutter.