Config endpoint assumes path will not end with `/`

[msavy]

A valid URL such as http://localhost:8080/apiman/ in config.json5 endpoint (and possibly others) causes 'double forward slash', as UI code adds a / to the end of endpoint.

For example: http://localhost:8080/apiman/ causes http://locahost:8080/apiman//<rest of path>

Acceptance criteria

• URL should be normalised so that slashed and non-slashed URLs work.

Possible solutions

• Normalise by always trimming the slash from endpoints in #get<whatever> URL endpoints, (consistently throughout codebase so all config properties do the same).
• Use built-in URL class to join URLs instead of doing it by hand new URL('/whatever', 'http://localhost:8080/apiman/').href

new URL('/whatever', 'http://localhost:8080/apiman/').href 
'http://localhost:8080/whatever'
new URL('/whatever', 'http://localhost:8080/apiman').href 
'http://localhost:8080/whatever'

Might be good to hide some of this behind the scenes, with an extra service endpoint like

getEndpoint(path: string): string { 
   return new URL(path, getEndpoint());
}