refractor: accounting module (#300)

* Refractor components.
* Add resolvers.
* Add relative router links.
* Replace mat-icons by font-awesome icons.
* Add comments.

Fixes: #298

src/app/accounting/accounting-rules/one-of-the-fields-is-required.validator.ts

@@ -0,0 +1,11 @@
+/** Angular Imports */
+import { FormGroup, ValidatorFn, ValidationErrors } from '@angular/forms';
+
+/** Validates that one of the fields for debit or credit is filled for a given accounting rule type. */
+export const oneOfTheFieldsIsRequiredValidator: ValidatorFn = (accountingRuleForm: FormGroup): ValidationErrors | null => {
+  const accountToDebit = accountingRuleForm.controls.accountToDebit.value;
+  const debitTags = accountingRuleForm.controls.debitTags.value;
+  const accountToCredit = accountingRuleForm.controls.accountToCredit.value;
+  const creditTags = accountingRuleForm.controls.creditTags.value;
+  return ((accountToDebit || debitTags) && (accountToCredit || creditTags)) ? null : { 'error': true };
+};