Update login / signup buttons copy and font

[yaelirub]

WPiOS issue: wordpress-mobile/WordPress-iOS#15703
This PR updates the copy on the login / signup button from "Welcome to WordPress.com" to "Log in or sign up with WordPress.com"

Since the title is more lengthy, this PR also updates the title label font of both buttons from title3 to body per design

To Test:

1. Update the WordPressAuthenticator path in WPiOS to this branch
2. Be signed out
3. See copy as described
4. See font is body as described
5. Continue to next screen and see button title fonts are unchanged (title3)

[mattmiklic]

This looks fine in English at the default font size, so I'm going ahead with my 👍 here -- but it would be good to get a build so we can look at longer translations, and non-default system font sizes, to make sure the buttons adapt gracefully.

[mattmiklic]

I tested the build with dynamic font sizes enabled and in some other languages. I think we are generally fine to launch this as-is. I do have one suggestion for improvement but as it's not a regression, it can be a separate issue.

At the current font sizes, some of the buttons are split into two lines. German is a good example. The same is true when dynamic font sizes are enabled. When the text wraps to two lines, we'd benefit from having some top and bottom padding in the button element to keep the text from getting uncomfortably close to the edge of the button shape.

[ScoutHarris]

FWIW, there is an open issue to redo that button because of wrapping issues. #501.