Add BP styles for the Welcome Screen

[mercime]

1. We've been updating and playing catch-up to fix wrecked BP welcome screens (aka About Page) layouts whenever a new version of WP has changed/removed styles at least least twice, to wit: ​https://buddypress.trac.wordpress.org/ticket/5561 and ​https://buddypress.trac.wordpress.org/ticket/6582.

2. Then recently, karmatosed brought to our attention that the nice layout we had for BP 2.4.0 as shown here ​https://buddypress.trac.wordpress.org/attachment/ticket/6651/2-4-welcome-screen-final.png now needs vertical spacing between feature images and headings in WP 4.4+.

3. In addition, BP 2.4.0 supports WP 3.8.0 through WP 4.4+. The layout of our About Page is wrecked in WP 3.8+/3.9+. See screenshot ​https://www.dropbox.com/s/a1364kitjzggayy/WP-39-BP-25.png?dl=0

We cannot rely on WP's ever-changing styles for this page as evidenced above. WP core has done what they want with the styles for what they need to show in the project’s About page, and rightly so. This ticket will create/adopt/adapt styles for BP's About Page to avoid any more breakage in new WP versions as well as in the older WP versions that we still support.

[DJPaul]

FYI, BP 2.5 will require WP 3.9. Also, my proposed email change currently has a dependency on WP 4.0 though we may be able to work around that.

[hnla]

So in a nutshell we have been relying on inheriting many styles for our screen layout from WP styles, WP changes those styles drastically and our layouts collapse causing unnecessary time and effort redoing.

If WP re-factors their screen so drastically according to what they need for any given version and that is never likely to suit our requirements then yes we should describe explicit styles to manage our screen layout.

[mercime]

Thank you both for your feedback :)

@DJPaul Yes, I checked on both WP 3.8 and 3.9 as I was working on the styles for #6803.

@hnla Exactly!

Will be working on the patch this weekend.

[mercime]

We need this patch for the 2.5.0 Welcome screen #6886 so that the layout would show well in WP 3.9+. This would also ensure that our layout will be future-proof against changes made to WP core's about.css for 4.5.0.

[mercime]

In 10617:

Update stylesheets for the Welcome Screen.

Provide style support for our Welcome Screen in the older versions
of WordPress which we still support and ensure that our layout doesn't
break when WP core's about.css is changed.

Fixes #6826.