Re-applying: Unify on using standard OOUI 'linkExternal' icon, which is
directly replacing Vector's image without relying on ResourceLoader.
This time we have lo- and hi-dpi optimized icon.
Please note that the icon features standard link color `#36c` which is
implemented in a later step. Due to the small icon size and the fact
that current external link icon is also not using the legacy color, this
seems acceptable to move forward with.
Also using relative `em` instead of `px` to support user text zoom
capabilities and introducing a `@size-indicator` variable to be replaced
by WikimediaUI Base variables later.
Alternative to I49de3bfff45.
Replacing images and image names to make quicker lookup with icon
collection possible future-facing.
Note that CSSJanus is flipping the `ltr` string in the background image
rule to `rtl`, therefore making sure that both icons are available.
Bug: T261391
Change-Id: I1c3cfb4cd83fe4d5d08c06b84cdc192cb90c0dfe
