[feat] Support 'match all' forwarding

## Describe the feature

For context, see #319.

`forwardemail.net`'s wildcard rules are 'fall-through': if any 'specific' rule is matched, wildcard rules are not respected.
A small minority of email forwarding providers' wildcard rules are 'match-all': wildcard rules are always respected, i.e. they disregard 'specific' rules entirely.

It would be helpful if `forwardemail.net` could support this pattern on an opt-in basis.

## Example (Paid plan)

Note: I am a free plan user, I don't know what the UI for paid users looks like.

User creates a 'simple' forwarding rule which forwards `hello@example.com` to `first@gmail.com`
User creates a 'match-all' forwarding rule which forwards all emails on `example.com` to `second@gmail.com`

Emails addressed to `hello@example.com` should be received by both `first@gmail.com` and `second@gmail.com`

## Example (Free plan)

```
forward-email=hello:first@gmail.com,*:second@gmail.com
```
or, equivalently
```
forward-email=hello:first@gmail.com
forward-email=*:second@gmail.com
```

Emails addressed to `hello@example.com` should be received by both `first@gmail.com` and `second@gmail.com`.

## Tasks

- [ ] Definition of new forwarding rule type 'match all'
- [ ] User interface for creating 'match all' rules on paid plan
- [ ] Extension of forward-spec string syntax to enable specifying 'match all' rules without ambiguity
- Proposed 'match all' syntax: `*:[recipient address]`
- [ ] Implementation of 'match all' rule handling
- [ ] Documentation
- [ ] Unit tests