For font awesome icons I had only supported one of alt/title. This was because (subjectively) we have
done a terrible job of providing meaningful alt text for icons. This is not a good idea and not backwards
compatible so I have put it back again.

Fingers crossed I did not break 1000 behats.

image_icon is a better name because there are some valid uses for these other than for activity icons.

E.g. Old JS code that is not worth re-writing.

Because we can use svgs always - we got rid of different size icons for file types.

Match the expected output for unit tests.

Use font awesome svg icons for files in the file picker. Move the old icons to bootstrapbase.

Also change filepicker to use scaled svgs instead of sets of custom pngs. The svgs are taken from font-awesome.

Always use the pix icon helper in templates.

Activity icons always use the standard icon renderer.

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).