Fix #1213 - improve render warning behavior

[sc1f]

This PR fixes #1213 by improving the behavior of perspective-viewer's render warnings, which display when the chart is attempting to render a dataset that is larger than specified bounds and might have performance implications.

As outlined in #1213, render warnings would keep popping up even after a user has dismissed them, and it becomes an annoyance for the user. Additionally, the presence of the "Close" button allows there to be a state where the user has closed the warning that a subset of the data is being displayed, but if the viewer's config is not changed then there are no other indications that the user is looking at a subset of the data and not the whole.

This PR changes the behavior of the render warning in the following ways:

• Render warnings are now persistent per-plugin: after a user clicks "Render all points", the warning will not appear again for that plugin, even after the user switches to a different plugin.
• The "Close" button has been removed, so that there is always a warning present when the user is looking at a subset of data.
• A new render_warning option has been added to each plugin config, which allows the user to specify whether the warning should be shown at all (defaults to true):

const plugin = window.getPlugin("d3_heatmap");

// Change the bounds at which the warning should appear
plugin.max_cells = 10000;
plugin.max_columns = 200;

// Or disable the warning entirely
plugin.render_warning = false

• A new set of tests around the render warning has been added to several chart types in perspective-viewer-d3fc.

[texodus]

Looks good!

I think this implementation is an improvement over whats on master currently, a few things stick out to me re: the UX itself, that I'd like to answer in a future PR:

• The X ("dismiss the warning") button does indeed remove any indication the visualization is compromised, but the actual current implementation crops the top 30px of the view. I can buy not wanting this to go away, but if its going to be persistent I think it needs a redesign - maybe integration with the upcoming status bar?
• Is render_warning keyword a more obvious implementation of this feature than, e.g., max_cells: Infinity ? In essence, max_cells is a conceit to Perspective's somewhat asymmetric performance w.r.t. columns vs rows, so I can imagine a future simplification of this feature that only exports max_cells. At minimum, render_warning and max_* are exclusive and might benefit from some validation (and maybe also default to true 💯 ). Obviously, as this is a developer facing API, we have some room to iterate here!