Implement a "service" layer to manipulate table objects

[bhearsum]

Discussed with :bhearsum while implementing [1]. In a few words, the service layer is represent the whole business logic of a process, by dividing the business process into smaller ones. The smallest ones are then the table objects.
The idea of the back-end architecture would be the following: The web layer takes data out of the HTTP request and passes them to the Service. The service is then in charge of delegating to all the tables, and making sure data is consistent across tables

Here are some call diagram examples:

• Add a new scheduled change [2]
• Update a condition [3]
• Update a scheduled target value [4]

[1] #151
[2] https://www.websequencediagrams.com/?lz=dGl0bGUgQWRkIGEgbmV3IHNjaGVkdWxlZCBjaGFuZ2UKClMACghSdWxlc0FQSVZpZXctPgAJDlNlcnZpY2U6IGFkZF9uZXcocnVsZV9pZCwgcnVsZXNfdmFsdWVzLCBjb25kaXRpb25zLCBhdXRob3IpAFIPAEMHAFEQVGFibGU6IGluc2VydABZBQBLCgBBCG5vdGUgcmlnaHQgb2YgACsVYWxzbyBzYXZlcyB0aW1lc3RhbXAAgVMPAGUFAIFHGQCCFwlfAIFbBwCCERAAgSoWQwCBbwkAgTsOAD4RAIIXDCk&s=rose
[3] https://www.websequencediagrams.com/?lz=dGl0bGUgVXBkYXRlIGNvbmRpdGlvbnMKClNjaGVkdWxlZFJ1bGVzQVBJVmlldy0-AAkOU2VydmljZTogdQA8BV8ANwoocwA4CF9ydWxlX2lkLABUCywgYXV0aG9yKQBYDwBJBwBYD0MAgQ4JVGFibABlCQBFHgA1JgBEDl9sYXN0XwCBFQYAgS0UAIEtCA&s=rose
[4] https://www.websequencediagrams.com/?lz=dGl0bGUgVXBkYXRlIHNjaGVkdWxlZCB0YXJnZXQgdmFsdWVzCgpTABEIUnVsZXNBUElWaWV3LT4ACQ5TZXJ2aWNlOiB1AEkFKABFCV9ydWxlX2lkLCBydWxlc18AUAYsIGF1dGhvcikATw8AQAcAThBUYWJsACkz&s=rose

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1313742, original comment by @JohanLorenzo)