Construct all the server call URLs using string concatenation

Instead on the relativeURL method.

This is because the relative URL appraoch does not work on iOS even though the documentation says that it should

Relative URL doesn't work: e-mission/e-mission-docs#732 (comment)
Appending strings does work: e-mission/e-mission-docs#732 (comment)

Related updates:

• https://github.com/e-mission/cordova-connection-settings/releases/tag/v1.2.3
• https://github.com/e-mission/cordova-server-communication/releases/tag/v1.2.4
• e-mission/e-mission-phone#834