Sylius: Channels urls use `http`

[nenadalm]

Sylius version affected: master

Description
Channels can have only http urls - not https. Example: https://github.com/Sylius/Sylius/blob/281d33c1c7b3b83e9ac420e7be917e4632cb2593/src/Sylius/Bundle/ShopBundle/Resources/views/Email/verification.html.twig#L6. It would be nice if it was possible to configure https.

[pamil]

This surely needs some customisation option. My first idea would be to allow for http:// and https:// prefix in channel's hostname and falling back to http:// if not provided for backwards compatibility.

[Takeno]

What about let admin to decide it for each channel? I think a simple flag with Use https is enough to cover all cases.

[Prometee]

Because all websites on internet should provide https now, maybe it's good to change the default http to https and provide a service allowing to change this behaviour (ex: for dev/test env).
This become annoying in some cases like when we have to override all content blocks templates used for mails because they are internally using the string http://.

[loevgaard]

I have created a very simple PR that I believe fixes this. I totally agree with @Prometee that so many big orgs (i.e. Google) encourages the use of https over http, so it doesn't make sense to use http as default, rather https should be the default (which is what I have done in the PR).