[MIG] web_m2x_options_manager: Migration to 15.0

- Fix postprocess inherit (removed from v15), and use _postprocess_tag_field
for create/edit options: In v14 it worked with inheriting the "postprocess"
method because in each recursive call it passed a section of the view, in the
way that it went through all the child nodes of the root node, and passed through
the fields, and now in v15 the method "_postprocess_view "it just does the while,
and we never go through the nodes.

- Fix pylint errors: xml-deprecated-tree-attributte, attribute-string-redundant,
translation-positional-used

web_m2x_options_manager/tests/test_m2x_create_edit_option.py

@@ -4,11 +4,11 @@
 from lxml import etree
 
 from odoo.exceptions import ValidationError
-from odoo.tests.common import SavepointCase
+from odoo.tests.common import TransactionCase
 from odoo.tools.safe_eval import safe_eval
 
 
-class TestM2xCreateEditOption(SavepointCase):
+class TestM2xCreateEditOption(TransactionCase):
     def setUp(self):
         super(TestM2xCreateEditOption, self).setUp()
         ref = self.env.ref