Save notification url as relative instead of absolute

[AlexAndBear]

Description

Previous to this fix, absolute URLs were passed to the notification app.
This could cause some CORS issues, hence we now use relative ones.

Related Issue

• Fixes https://github.com/owncloud/enterprise/issues/4250
• Related PR in the notification app is required to ensure that links are correct in emails: Add command to repair notifications and properly handle mail sending … notifications#333

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[phil-davis]

Test changes LGTM - exactly the changes that are expected.