Make ee_compat_check work on other remotes and branches
Right now ee_compat_check can only use GitLab.com CE and EE repositories
and only checks against a target branch of master.
The EE repository can now be overriden via the EE_REPOSITORY_URL variable.
In addition, the EE target branch will be derived from the CI variable
CI_MERGE_REQUEST_TARGET_BRANCH_NAME. If the variable is blank or
master, then the EE target branch will be master. Otherwise, if the
merge request targets some other branch, it will append -ee as the
target branch. For example, if a merge request targets 11-7-stable,
then ee_compat_check will check 11-7-stable-ee.