Allow setting the user-agent string for the internal browser

[andyearnshaw]

A lot of sites I follow don't have responsive designs, instead opting to redirect to a mobile version of the site for mobiles. The problem with the desktop versions is that they usually have sidebars and other stuff that gets in the way of the content.

Emulating a mobile browser by changing the user agent string would get us content that fits nicely inside the reading pane.

[stale]

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