migrating NXRM2 to NXRM3 automatically enables legacy content URLs

According to the requirements in ---NEXUS-9976--- a migrated 3.x installation should automatically support the old legacy URL patterns. For v2 content URLs to work, however, a system property must be defined org.sonatype.nexus.repository.httpbridge.internal.HttpBridgeModule.legacy=true, and this isn't happening automatically.

Acceptance

• 2x-3x upgrade should automatically enable the v2 content URLs
• implementing this as a capability that gets injected and activated in Nexus 3 on migration start would be great! - otherwise if a property is used, inject it into nexus.properties under the work directory. Support still needs to have a way to tell this is enabled ( currently we just look in nexus-default.properties or nexus.properties ) and the end user still needs to have an easy way to tell as well ( presence of property in nexus.properties or capability in UI )
• Minimize upgrade impact to users that have already migrated and made decisions about turning this feature on or off

Notes

• There's value in having this as a capability
• While you're in there, make a mental note if you spot any obvious signs that a given NXRM3 instance (or individual repos) were migrated from NXRM2