Make it easier to discover and use EE features

We have only 2% of the people upgrading. Partially that is because CE is a great product and that will stay that way. But talking to with users I've found 3 things:

1. They are not aware of the features EE offers
2. They are hesitant to swap out CE with EE
3. They think the purchasing process is tedious (license per email, etc.).

The solution would be to:

1. Make the EE download the default https://gitlab.com/gitlab-com/www-gitlab-com/issues/1336
2. Give the EE download all the CE features when there is no subscription (EE subscriptionless)
3. Promote EE features in EE when there is no subscription ('Try LDAP group sync 30 days for free')
4. Have a free 30 day trial form in the product (only needs an email address and company information, no creditcard)
5. Have an EEP order screen in the product (after your trial expired, no license download needed, it works completely within the product)

Keeping in mind that:

1. For people that just want the CE features (EE subscriptionless) there are no drawbacks, no nag screens, etc.
2. We make it a single path: download => use as CE => see EEP feature => start trial => get EEP subscription. So no selection screen after you installed it, always go through a trial, and always EEP.
3. When the trial expires your are prompted to purchase EEP but you can opt to revert back to CE features.
4. When a subscription expires we have the same behaviour as currently when a subscription expires.
5. Apart from that we have to see if we can make https://docs.gitlab.com/omnibus/update/README.html#from-community-edition-to-enterprise-edition simpler.

/cc @JobV @markpundsack @stanhu @ChadMalchow @timanglade

@sytses I think this is a great flow. As you say, no downsides. For those that absolutely must have open-source only software, we'll have a link available (but severely downplayed) that lets them download it.

/cc @mydigitalself

Since the majority of this work is going to happen in the EE codebase, I'm going to move the issue to that project.