ui: Use "Source Sans 3" font for most UI text

[chrisbobbe]

Since Source Sans 3 is a variable-weight font, we'll have to take some extra care not to regress on support for the user's "use bold text" accessibility setting. We've been doing that by styling text with a TextStyle object that comes from weightVariableTextStyle (or is merged with one that comes from that). Pass a BuildContext to that function to let it conditionalize on that accessibility setting.

One might normally set a "default font" for the app without using TextStyle objects: by setting fontFamily or fontFamilyFallback on an app-wide ThemeData. But ThemeData doesn't also have a fontVariations field, which controls text weight for weight-variable fonts.

Discussion: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20text.20all.20weird/near/1633648