[client] Name the two config readers for what they do

ReadConfig and GetConfig differed in one thing — what happens when the file is
absent — and neither name said which was which:

- ReadConfig      -> ReadOrGenerateConfig  (reads it, or generates one in memory)
- GetConfig       -> GetExistingConfig     (reads it, or fails)

Three comments went with them:

- GetConfig's said "return with Config and if it was created. Errors out if it
  does not exist", which described a bool it does not return and a creation it
  never performs.
- ReadConfig's explained that it does not write, which is what a reader is
  supposed to do anyway.
- Server.getConfig's said it "errors out if it does not exist", which it does
  not — it resolves a default config, and now provisions the identity too.

client/mobile/profile_manager.go

@@ -174,7 +174,7 @@ func (pm *ProfileManager) LogoutProfile(id string) error {
 		return fmt.Errorf("profile %q does not exist", id)
 	}
 
-	config, err := profilemanager.ReadConfig(configPath)
+	config, err := profilemanager.ReadOrGenerateConfig(configPath)
 	if err != nil {
 		return fmt.Errorf("read profile config: %w", err)
 	}