- A missing gitlab_schema.graphql file can cause
  unrelated and hard to debug issues.
- This can be overwritten with GL_IGNORE_WARNINGS=1

We do not use it, it can be replaced by introspection queries, and
it complicates development.

The `gitlab:graphql:schema:dump` rake task now generates the file
in the `./tmp` directory.

The documentation is updated to mention the impact on front-end
development.

This is part of the work to implement the consensus reached in the
[RFC][1] to prefer named exports over defalt exports.

Future iterations will migrate existing default exports to named
exports.

The bulk of the changes in this commit were performed with the following
script:

```bash
set -u

changed-files()
{
    local trunk="origin/master"
    local file=
    local committed_files=$(git diff "$trunk"... --numstat \
        | awk '/\.(js|vue)$/{ print $3 }')
    local changed_files=$(git diff --numstat \
        | awk '/\.(js|vue)$/{ print $3 }')
    for file in ${committed_files[@]} ${changed_files[@]}; do
        if [ -f "$file" ]; then
            echo "$file"
        fi
    done
}

git grep -lz "import/prefer-default-export" -- '**/*.js' \
    | xargs -0 perl -0pi -e \
    "s/[^\n]*eslint-disable[^\n]*"\
"import\/prefer-default-export[^\n]*\n//mgs"

git grep -lz "rfcs/-/issues/20" -- '**/*.js' \
    | xargs -0 perl -0pi -e \
    "s/[^\n]*rfcs\/-\/issues\/20[^\n]*\n//mgs"

yarn prettier --write $(changed-files)
```

The script is [idempotent][2], to help make it easier to keep
up-to-date.

[1]: https://gitlab.com/gitlab-org/frontend/rfcs/-/issues/20
[2]: https://en.wikipedia.org/wiki/Idempotence

Also sets up helpers needed for mock server