CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14520

This backports two EE specific changes made to the schema changes. We do
not yet backport the EE prepend, as this will/can be done when
https://gitlab.com/gitlab-org/gitlab-ce/issues/58170 is implemented.

This backports all EE schema changes to CE, including EE migrations,
ensuring both use the same schema.

== Updated tests

A spec related to ghost and support bot users had to be modified to make
it pass. The spec in question assumes that the "support_bot" column
exists when defining the spec. In the single codebase setup this is not
the case, as the column is backported in a later migration. Any attempt
to use a different schema version or use of "around" blocks to
conditionally disable specs won't help, as reverting the backport
migration would also drop the "support_bot" column. Removing the
"support_bot" tests entirely appears to be the only solution.

We also need to update some foreign key tests now that we have
backported the EE columns. Fortunately, these changes are very minor.

== Backporting migrations

This commit moves EE specific migrations (except those for the Geo
tracking database) and related files to CE, and also removes any traces
of the ee/db directory.

Some migrations had to be modified or removed, as they no longer work
with the schema being backported. These migrations were all quite old,
so we opted for removing them where modifying them would take too much
time and effort.

Some old migrations were modified in EE, while also existing in CE. In
these cases we took the EE code, and in one case removed them entirely.
It's not worth spending time trying to merge these changes somehow as we
plan to remove old migrations around the release of 12.0, see
https://gitlab.com/gitlab-org/gitlab-ce/issues/59177 for more details.

- create tables for storing issue trackers properties
- add model and basic logic & spec