Bug 12539: Follow up to fix fallback theme

Bug 12539: Follow up to fix fallback theme

The is a problem in the logic, fallback must be:
1. Requested (theme, lang, tmpl)
a. return    (theme, lang, tmpl), if not
b. return    (fallback, lang, tmpl), if not
c. return    (theme, 'en', tmpl), if not as last resort
d. return    (fallback, 'en', tmpl)

Previous patch missed option 'c', worked for CCSR
but not bootstrap, sorry.

Signed-off-by: Petter Goksoyr Asen <boutrosboutrosboutros@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>