Default vue_issuables_list feature flag to on

Tracking of :vue_issuables_list feature flag.

These issues are waiting verifications on production:

• #217569 (closed)
• #233725 (closed)

Checklist:

• Default :vue_issuables_list to on
• Remove :vue_issuables_list feature flag code from the codebase
• Remove vue_issuables_list feature flag via Chatops for production, staging and dev