Add xref and yref to legends

[hannahker]

Summary: Add support for container-referenced positioning for plot legends. See demo CodePen here: https://codepen.io/hannahker/pen/wvEZZvN

API changes: yref and xref parameters are added to layout.legend.

Expected behaviour:

• The legend may be positioned relative to the full container area if layout.legend.yref or layout.legend.xref are set to 'container'.
• If a legend has container-referenced positioning, then space will be added to the _fullLayout._reservedMargin accordingly, meaning that legends may be automatically positioned to avoid components such as automargined axes.
  • If xref='container', then margin space will be added to the _reservedMargin on the left or the right of a plot area (depending if x > 0.5)
  • If yref='container', then margin space will be added to the _reservedMargin on the top or bottom of a plot area (depending if y > 0.5).
  • If both xref='container' and yref='container', then margin spacing will be added to the top or bottom if orientation='v' and left or right if orientation='h'.

The defaults for yanchor, xanchor, y, and x have also been adjusted for cases where container-referencing is applied.

[archmoj]

Great PR 🥇 thanks to @hannahker.
Looking good to me.
Over to @alexcjohnson for 🔎 second 🔍 review.

[alexcjohnson]

Looks great! Just one more remnant of moving the min/max logic to defaults. 💃