In templates we determine checkboxes are disabled by using the following
`if` clause,
```
{{#if (or (and is_muted notification_setting) realm_setting_disabled)}}
disabled="disabled"
{{/if}}
```
and it is more intuitive to do such calculation in javascript code, so we
added an `if_disabled` attribute in `settings` context which replaces
logical operations from `if` statement.
So for non-notification settings, it is
```
is_disabled: check_realm_setting[setting]
```
where check_realm_setting[setting] is same as realm_setting_disabled.
and for notifiaction settings it is,
```
ret.is_disabled = check_realm_setting[setting] || sub.is_muted;
```
Fixes#13134 as the last commit in the series for this issue.
Solves the "The (?) should just be a target=_blank link to
/help/message-a-stream-by-email." part of the issue.
As a result, a bunch code managing the email hint popup can be deleted,
together with a node test for that.
Pragati Agrawal
Tim Abbott
Mateusz Mandera
Anders Kaseorg