Force BR to always be display:inline

[poire-z]

BR shouldn't be handled just like any block element (a height applied to it is not ensured by Firefox or Calibre).
We get more similar results with Firefox/Calibre by just forcing all BR to be display: inline.
Revert a bit of 36ae6cc (from #337) (but let the logic in for <empty-line>).

More details in koreader/koreader#6069 (comment) and #172 (comment).

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Thought about removing the height in epub.css:

/* Element added for each empty line when rendering plain txt files */
empty-line {
    height: 1em;
}

to keep lines aligned with .txt documents - but they use PRE for each line, which has some top and bottom margins, so preventing any constant line-height alignment.

But just noticed that if you "Clear all external styles" with txt document, you get an alternate rendering, without monospace fonts, no margin for PRE, no height for empty-line, so something that I find more readable and that has constant line-height aligmnent:

I'd like to rename that menu item: Clear all external styles to just None.
That long sentence is a bit unclear about what it does, what are external styles, why the plural (ok, it's many styles in a single user agent CSS), and styletweaks still work when this is checked.
Thoughts?

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This change is 

[Frenzie]

Yes, I like "None".