Linker: Render selflinks as href-less classed <a>s, not <strong>s

Linker: Render selflinks as href-less classed <a>s, not <strong>s

Self-links are still semantically links, and representing them as <strong>s
is inelegant and more important a real pain to work with, especially in
contexts where they may change state (like inside an editor).

Instead, render them as <a>, with no href to avoid user agent style over-
rides and with a class to style them as before, named 'mw-selflink' to go
with 'mw-redirect'. This allows much easier adjustment later. The old CSS
class 'selflink' is retained for backwards compatibility, but deprecated.

Bug: T160480
Change-Id: If058843924c3b30c116df2520aef93a004d98a5d