Fix issue with missing volumeMount in ent portal statefulset

[rewsmith]

Description

Added missing Enterprise Portal statefulset volumeMount /opt/portal/themes without which Portal v1.2.0 will crash as it can't write to the file-system

Related Issue

Motivation and Context

Test Coverage For This Change

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[rewsmith]

@andrei-tyk @buraksekili Thanks for the reviews. Can this now be merged?