Updates to flat button behaviors

[jtamiace]

Based on a Slack thread

Issue summary: We have inconsistent ways of handling the alignment of flat button components in our products. Should the text or component be aligned to a page container?

A couple of takeaways (copy pasted)

1. Add the rules “For flat buttons: align button text, not the button component to container content boundaries; in button rows, keep 8px margin between components” to general Button Patterns documentation
2. Add a new Boolean prop on flat buttons offset which, when set to true, adds -8px left and right margins, which will align the button based on text rather than outer boundary

Implications for 2 current PRs https://github.com/learningequality/kolibri-design-system/pull/16 https://github.com/learningequality/kolibri-design-system/pull/21