WebInstaller: Don't call getSoftwareLink()

WebInstaller: Don't call getSoftwareLink()

This gets the web installer working without the mysql or mysqli
PHP extension, which is not needed for other database systems.

* Avoid "lego" construction: put link text directly in the messages,
  which because the messages are parsed as wikitext, is easy to do.
* Put URLs in separate, ignored messages to avoid duplication in
  Database*::getSoftwareLink().
* Because I had to touch a MySQL-related i18n message, I figured
  this would be a good time to address bug 28281 (regarding MySQL
  forks and Special:Version).
* Other miscellaneous message changes, such as recommending the
  mysqli PHP extension over the old, deprecated mysql extension.

Bug: 28281
Bug: 58167
Change-Id: I6dbc9d32c8c2d8f233a67d574229b80444885aea