[FieldFormatters] Relative URL formatter inconsistencies with non-empty basePath or custom space

[Dosant]

Kibana version: 7.12.0

Describe the bug:

Relative URL inside URL field formatters behave differently depending on if Kibana is running on a non-empty basePath or/and inside a custom space.

URL	Kibana empty basePath	non-empty basePath	custom space
/test/hello	http://localhost:5601/test/hello	http://localhost:5601/vij/app/test/hello	http://localhost:5601/vij/s/custom/app/test/hello

Any additional context:

So it looks like it is implemented in a way, that field formatted expects relative URL to include any non-empty basePath (including custom space prefix).

For example, assuming we are in a space and it is available at http://localhost:5601/vij/s/custom/, then to have a relative URL we need to use: /vij/s/custom/test/hello.

Such approach breaks relative URLs in URL field formatters when launching Kibana on a different basePath or when transferring/sharing/copying index pattern into a different space.

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)