When we check if MR is mergeable we also check the CI pipeline.
Since we are creating the pipeline asynchronously via
`MergeRequests::AfterCreateService`, there's a possible race
condition wherein the pipeline isn't created yet but we already
checked if MR is mergeable. That can result to inaccurate outcome.

To ensure that whenever we check if MR is mergeable, we introduced
a new internal state called `preparing`. When the MR is in this
state, it means it's still being prepared (e.g. creating a pipeline
) and it's not mergeable.

Once the preparation is finished, we mark the MR as unchecked so
we can check its mergeability when the user views the MR page.