Change plaintext mail views to use alternateBase as background and style
hyperlinks correctly.

Other minor changes:
 const a static string
 remove a stray space

Note:
 The whole tab should actually be using alternateBase for its colour,
 however this requires a stylesheet it seems - and I'm lazy.
Make sure that the text on tags contrasts, while there make them a
little less neon green.
Clean up two redundant/strange bool return statements.

No functional change.