Introduces the concept of Prerequisites for a CI build.
If a build has unmet prerequisites it will go through the
:preparing state before being made available to a runner.

There are no actual prerequisites yet, so current
behaviour is unchanged.

In a few models we define ActiveRecord enums that are redefined in EE
using the following pattern:

    enum :some_enum, {
      ...
    }.merge(EE_ENUM_VALUES)

This particular approach is problematic to deal with, because it
requires that we `prepend` and EE module _before_ defining the enum.
This typically translates to the `prepend` being the first line in the
model in EE, but this can easily lead to merge conflicts when developers
add more `include` and/or `prepend` lines.

As part of https://gitlab.com/gitlab-org/gitlab-ee/issues/8244 and
https://gitlab.com/gitlab-org/gitlab-ee/issues/8241 we are moving
`prepend` to the last line in a file, reducing the chances of running
into merge conflicts. This poses a bit of a problem with the pattern
above, because this pattern does not allow us to move the `prepend`
further down a file.

To resolve this problem, we simply move the Hash value of the enum to a
separate class method. This method is defined in a separate module where
necessary, allowing us to use it like so:

    enum :failure_reasons, ::SomeModelEnums.failure_reasons

The method in turn is defined in a very straightforward manner:

    module SomeModelEnums
      def self.failure_reasons
        {
          ...
        }
      end
    end

This makes it easy for EE to add values without requiring the `prepend`
to be placed before the `enum` is defined.

For more information, see the following issues and merge requests:

* https://gitlab.com/gitlab-org/gitlab-ee/issues/8244
* https://gitlab.com/gitlab-org/gitlab-ee/issues/8241
* https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8424