Added 'clear' messages as a configurable option

[metju90]

We planned to persist the emails caught by this software but unfortunately one colleague was not aware of this and deleted all emails by pressing "clear".

This motivated me to open this PR, to have the "clear" button as a config option.

[sj26]

MailCatcher should not be used as a durable store for messages, it uses an in-memory database, so I'm not sure I'm keen on adding this as an option. What's your use case here; why don't you want emails to be cleared?

[damienfinck]

For my case: we use mailcatcher for recipe environments where many users have access. So it happens that users click on it and delete everyone's mails. So I removed the "clear" button and put a limit to keep only the last 300 messages. We use mailcatcher with persistent storage in Docker containers.