Make Stock effect compare against previous day's close

[rbergen]

Description

This closes #662 by making the effect compare against the previous day's close instead of today's open. It does this both in the numeric change shown, as in the graph underneath it.

There is still one "peculiarity" that people may notice, which is that the data points in the graph include after-hour trading, and we don't show this in the coloring of the graph (Google and other sources tend to show after-hour trading in a different color).

This means that the last data point for a calendar day may be positive (green) while the (correct) day change is negative, or vice versa. If we want to change this then we need the stockserver to indicate in its response to the effect what data points are outside of regular trading hours.

Also, the default host for the stockserver changed from davepl.com to davepl.dyndns.org, to better handle IP address changes for Dave's Internet connection.

Contributing requirements

• I read the contribution guidelines in CONTRIBUTING.md.
• I understand the BlinkenPerBit metric, and maximized it in this PR.
• I selected main as the target branch.
• All code herein is subjected to the license terms in COPYING.txt.

[davepl]

Nice! Simple fix, doesn't mean it was simple to figure out :-)

[rbergen]

Thanks! The hard work was actually done in the back-end (stockserver), where it belongs, a few weeks ago. :)