net/vnstat: dashboard widget

[joeroback]

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Related issue
If this pull request relates to an issue, link it here: closes #5353

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Describe the problem
Vnstat dashboard widget

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Describe the proposed solution
Adds vnstat dashboard widget. Allows you to select interface, and period (hourly, daily, monthly, yearly)
It remembers the last selected interface/period
Preferences to select the update tick interval, defaulting to 5 minutes

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[sopex]

I would suggest these to be next to each other :)

Also, under no circumstances should a scrollbar appear. Lock the possible sizes a bit.

[joeroback]

i am trying to figure how to do that. the sizeToContent seems like its just a suggested size and almost all existing widgets have scrollbars and don't resize automatically to content, with the exception of the interface statistics widget.

there is a lot of text in this widget, so if its squashed to 1 column, it looks weird, but its not obvious how to require 2+ columns.

i've added an option to show/hide the avg rate column, which should help ppl who want a smaller widget, but still 4 columns don't really render under one column. open to suggestions, but also prefer not over engineer this simple and helpful widget

[joeroback]

ok i improved the height resizing, but still limited the sizeToContent to 1000px, which is more than all the other widgets currently, which seems to be 650px max. so scrollbar can appear in edges cases.

i also added another config option to exclude selected interfaces from the widget, with the defaults of enc0, pflog0, pfsync0. not sure if there is a better way to get the list of interfaces, currently i am getting them from the vnstat database vs the system interfaces. either choose has pros and cons