[FW][ADD] UI: odoo spreadsheets svg icons

[fw-bot]

Task: #4792034

This PR proposes introducing the o-spreadsheet icons into the Odoo documentation.

---

How this works

The Spreadsheet library in Odoo contains two QWeb template files for SVG icons:

• o_spreadsheet/icons.xml
• o_spreadsheet/o_spreadsheet.xml

These were used to generate an SVG sprite file (extensions/odoo_theme/static/img/odoo-spreadsheet-icons.svg), which can then be utilized by the SVG <use>element to call nodes by ID in the sprite.

This SVG sprite file is then included in the main page layout (extensions/odoo_theme/layout.html)

The RST :icon: role was updated to accomodate an os-* icon specifier.

The SVG sprite file can be generated automatically via a simple script outlined in this gist:
https://gist.github.com/samueljlieber/5ae7648a5ca08e0bfc8455861825e2ff

---

Why?

This PR attempts to scale an earlier method I used, the RST raw-html(raw) role includes the <svg> markup directly in the Global Filters doc (see this diff).

This was/is not scalable, is not usable throughout multiple docs, and will decay over time as the o-spreadsheet library is updated. This PR attempts to find a solution for using o-spreadsheet icons in documentation.

---

Example

With this implementation, o-spreadsheet icons can be used like so:

This is the :icon:`os-global-filters` :guilabel:`(global filters)` icon!

Forward-Port-Of: #13386

[robodoo]

[fw-bot]

This PR targets saas-18.2 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port