[IMP] portal, website, website_*: make footer content easy to customize

Previously, customizing the website footer was not very easy. This
commit will allow users to customize the footer like any other
snippet area.

There are several behavior changes, as mentioned below:

- Removed toggles 'Automatic Footer' and 'Payment Icons' from the
  Customize menu
- Language Selector was moved near the 'Copyright' section of the footer
  and features a dropdown (dropup technically) to select the language
- Moved some footer links from various modules to top menu
  List of the added menus / changed menus:
    (a) Resellers (new menu for previous footer link 'Resellers')
    (b) References (new menu for previous footer link 'Our References')
    (c) Forums (renamed from 'Forum', as a replacement of 'Forums'
        footer link, showing all the forums instead of 'Help', as there
        is no link available to all forums after this commit)
    (d) Live Support (new menu for previous footer link
        'Livechat Support')
    (e) Mailing Lists (new menu for previous footer link 'Mailing List')
    (f) Members (new menu for previous footer link 'Members')
- Links which were already available as a website menus (Presentations,
  Jobs, Events, News, Documentation, etc) are simply removed

Technical Notes:

- Introduced a new template called 'brand_promotion' in website.
  This will help to avoid adding redundant footer content (like
  copyright, language selector etc) from several modules (website_sale,
  website_event, website_quote, etc) while the only intention is to
  replace module page links.
- Removed summernote related fix in portal.less (caused by xpath)
  because footer is now customizable and added padding to structure
  instead of footer itself so that bg-color/img can be applied on
  whole footer.
- Few of the tours are improved to drag and drop snippets at proper
  places and not inside the footer.
- Used Flex in copyright portion of footer, so that text will always be
  horizontally centered (Some themes have different padding for buttons,
  etc, so if we don't align text dynamically, it has to be managed in
  particular theme and in particular layouts like full-screen / boxed.
  And if someone changes height of this portion, we have to re-design
  this portion again, etc).

task-38069
Closes https://github.com/odoo/odoo/pull/22298



Co-authored-by: Dharmraj <dja@odoo.com>