Dont assume icons are image tags and call Templates.renderPix to rebuild the entire icon.

We add a new theme config so the theme can say it supports font-awesome.
If this is true, the pix_icon renderer will call a mapping function to map
from the moodle style t/edit to a font-awesome style fa-cog icon name. Then the renderer
will either render an image tag for old icons - or an accessible font-awesome <i> tag.

This mostly works - but there are some places where we don't use the pix icon renderer, and
we directly create image tags with pix_url image sources. These will need updating (Atto icons,
drag and drop move icons, editpdf icons).

AMOS BEGIN
 MOV [unoconvwarning,fileconverter_unoconv],[unoconvwarning,admin]
AMOS END

Merge branch 'MDL-58023_behat_find_node_in_navigation_improvement_master' of https://github.com/gthomas2/moodle

In unit test, we write cache file often and
opcache might fail to detect changes. It will
be nice to invalidate opcode cache to ensure
we have the correct file_exists status