Add global layout configuration

[fluffy-critter]

Summary

Adds the ability to globally configure the default layout settings for entry.body/entry.more/etc.

Closes #424

Detailed description

The configuration object now takes an additional dict, layout, which provides defaults for layout methods. For example,

app = publ.Publ(name, {
    'layout': {
        'max_width': 768,
        'max_height': 1024
    }
})

Additionally, some fixes were made to how default attributes get applied to pure-HTML entries, and external images now use CSS to specify the container size instead of always applying max_width to the width attribute.

Developer/user impact

Test plan

Added a global max_width default of 768 to the test app, and verified that all of the larger-than-768xwhatever images were clamped to 768xwhatever, and that it could be overridden on a per-image basis if needed.

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