ActiveModel::Serialization is simple in that it recursively calls
`as_json` on each object to serialize everything. However, for a model
like a Project, this can generate a query for every single association,
which can add up to tens of thousands of queries and lead to memory
bloat.

To improve this, we can do several things:

1. We use `tree:` and `preload:` to automatically generate
   a list of all preloads that could be used to serialize
   objects in bulk.

2. We observe that a single project has many issues, merge requests,
   etc. Instead of serializing everything at once, which could lead to
   database timeouts and high memory usage, we take each top-level
   association and serialize the data in batches.

For example, we serialize the first 100 issues and preload all of
their associated events, notes, etc. before moving onto the next
batch. When we're done, we serialize merge requests in the same way.
We repeat this pattern for the remaining associations specified in
import_export.yml.

This brings a significant refactor to how
we handle `import_export.yml`, merge it with EE
and how we handle that for reader and saver.

This is meant to simplify the code, and remove
a ton of conditions to handle different models
of the structure.

This is also meant to prepare the structure
to extend it much easier, like adding `preload:`
or additional object types when needed.

This does not change the behavior of import/export,
rather unifies and simplifies the current implementation.

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.