Improve welcome dialog support for large fonts

Objective:

* Make the welcome dialog work with smaller fonts by increasing its
  size a bit, and also making its contents scroll vertically if needed

Changes:

ve.ui.MWBetaWelcomeDialog.js
* Move content into a scrollable and padded panel layout element

ve.ui.MWDialog.css
* Remove padding, now provided by panel layout

ve.ui.Dialog.css
* Remove min-height override, inherited value is fine
* Use max width and height instead of setting them directly

Bug: 52667
Change-Id: Ie35b041e13ef18ffcacef6d9e70439e4e1351e0a
Merge "Improve welcome dialog support for large fonts"