This commit renames the module document into attachment_indexation
to provide a more descriptive name and avoid confusion with
enterprise's documents module.

task: #1908896

This reverts commit 1dbb555a

Bring the sidebar attachment dropdown feature back.

This feature was removed due to be considered as useless,
but since some models can have attachement without a mail_thread,
this feature is still necessary.

task: #1908896

This rev. introduces robust helpers to use in the JS tests suite to
interact with DOM and components, and starts using them (almost)
everywhere.

All the helpers are exposed though testUtils.js.

There are 2 kinds of helpers:

1. Assertions
-------------
 * assert.containsNone, containsN, containsOnce check that the DOM
   (or a specific part of the DOM) contains a `selector`. It
   generates a correct error message automatically.
    ex: assert.strictEqual(form.$('.o_form_editable'), 1, "msg");
        -> assert.containsOnce(form, '.o_form_editable');
 * assert.isVisible, isNotVisible check that the DOM has an element
   visible or not. They also check that the element is actually in
   the DOM (before most tests didn't verify this).
 * assert.hasClass, doesNotHaveClass, hasAttrVAlue, check specific
   properties of a DOM element, and also validate that it is
   applied on a single existing DOM element (before most tests
   didn't verify this).
    ex: assert.notOk(form.$('button').hasClass('btn-primary'));
       -> assert.doesNotHaveClass(form.$('button'), 'btn-primary');

2. Utilities
------------
The goal of the utilities is to centralize the definition of many
standard components and interactions, ensuring that when we
refactor the JS framework, we do not need to change all the tests.

Existing mock utilities (addMockEnvironment, intercept, path,
patchDate, unpatch and fieldsViewGet) are moved to
'testUtils.mock.*'.
Existing DOM utilities are moved to 'testUtils.dom.*'.
New dom utilities are created for opendDatePicker, click,
clickFirst and clickLast. Helper `click` verifies that there is
exactly 1 element visible in the DOM you click on, `clickFirst`
and `clickLast` verify that there are more than one element on the
DOM.
 ex: form.$('button').click();
     -> testUtils.dom.click(form.$('button'));

New Form utilities: (testUtils.form.*)
 clickEdit, clickSave, clickCreate, clickDiscard, all clicks on
 the control panel buttons of the form.
 `reload` reloads the form data.

New modal, graph, kanban and pivot utils (testUtils.pivot.*,
testUtils.kanban.*, etc.).

New fields utils: (testUtils.fields.*)
 * editInput, editSelect: allow to change the value of a field,
   using a selector to identify it. They validate that the input
   exists and trigger the change event automatically.
 * editAndTrigger: allow to modify a field and trigger specific
   events after the value change
 * many2one (testUtils.fields.many2one.*)
   clickOpenDropdown, clickHighlightedItem, clickItem,
   searchAndClickItem: use a field name instead of a selector and
   do all the complex mechanism to open, filter and highlight
   many2one fields.

Joint work with aab, dam, ged, mge, svs and vsc.