selenium: Upgrade from webdriver v4 to v5

* Options no longer needed or no longer exist in wdio v5:
  - coloredLogs: Now always on. The underlying 'chalk' library can still
    be influenced via the FORCE_COLOR environment variable.
  - screenshotPath: Removed. Was already disabled in our config.
  - deprecationWarnings: Meh.
  - 'sync: true' – On by default when `@wdio/sync` is installed.
    The wdio v5 config generator doesn't recommend setting manually.

* The selenium.sh script was removed. It existed to start and stop
  chromedriver for local use by developers. This is now done by
  the wdio-chromedriver-service. In WMF CI, Quibble starts its own
  chromedriver (as optimisation, reused across gated repos),
  which is why the 'selenium-test' entry points remains and skips
  this.

* The wdio-mediawiki package now requires wdio v5 and Node 10.
  This doesn't affect extension repos because versions are pinned.
  Upgrade may happen at the earliest convenience.

* Several WDIO methods changed names or signature. Full list at:
  <https://github.com/webdriverio/webdriverio/blob/v5.13.2/CHANGELOG.md#v500-2018-12-20>
  Highlights:
  - browser.element() is now browser.findElement() with "$()" as alias.
  - browser.localStorage replaced by browser.setLocalStorage.
  - browser.deleteCookie() requires `name` param. To delete all at once,
    there is a new method browser.deleteAllCookies().
  - Commands that return data no longer wrapped in `{ value: … }`.
    Values are now returned directly.
  - Custom config keys are now under browser.config instead of browser.options.
    Renamed our username/password keys to be mw-prefixed, to avoid clashes
    and reduce confusion with similar config keys.
  - browser.click(selector) and browser.getText(selector) no longer exist.
    Use $(selector).click() or .getText() instead.

* Fix "no such alert" warning from specs/page.js by removing the apparently
  redundant code.

Bug: T234002
Bug: T213268
Change-Id: I908997569ca8457997af30cb29e98ac41fae3b64