Only set up FastGettext on initialization, even in development mode

[This fix is directly ported from alavetelitheme.]

Even on a very fast computer, there is a significant delay (a couple
of seconds) in processing every request in development mode.  This
is because the configuration of FastGettext's paths for finding
translations is done in a block passed to
Rails.configuration.to_prepare, which causes the block to be run
before every request by the Rails Reloader in developmetn mode.

It seems to be work fine just running the FastGettext calls on
initialization in development mode, however - customizations in
the theme's locale-theme directory still have effect in
development mode even after the reloader has reloaded classes
before handling subsequent requests.  So, I think this change
should be safe.  This commit introduces that change, having the
effect that development mode is much more responsive.
Remove empty template override.
Move images into assets
Move stylesheets to assets
Add theme asset dirs to config.assets.path.
Update image references to use asset image_tag calls
Remove obsolete example file.
Update stylesheet inclusion for asset pipeline.

Some @imports are superceded by the application.css manifest. For the
blueprint imports, use the solution suggested at

http://stackoverflow.com/questions/7769799/configuring-blueprint-in-rails-3-1

for including the stylesheets in the relevant context.
Convert stylesheets to scss

So that we can use image-url
Fix path.

Seems clearer and more robust to specify it relative to rails root.
Add stylesheets to precompilation list
Fix image link.

Previously this pointed to a non-existent image. That's a problem with
the asset pipeline, so point it to what I assume was the intended
target.
No need to symlink anymore.

Superceded by asset pipeline.
Fix fancybox css link.

Corresponds to https://github.com/mysociety/alaveteli/commit/5c19b4ba8d875e2aca7975e0cc604b3c441bda47
Merge remote-tracking branch 'mysociety_github/development-performance-fix'
Merge branch 'feature/asset-pipeline'

Conflicts:
    assets/stylesheets/custom.css.scss
    assets/stylesheets/main.css.scss
Revert "No need to symlink anymore."

This reverts commit 236716a0e37322a5563d9a0e6aea386e6f4e9827.
The symlink is required for the new donation microsite.
Override header rather than layout

All the customization seems to be in the header partial, so only
override this in order to keep this theme maintainable.
Use popup.css.scss for no_chrome layout

This layout is used when rendering the registration page in a popup -
before using the asset pipeline, the main.css file from Alaveteli was
rendered. As this is generally overriden by the theme main.css now,
retain it as popup.css and use it to render in the modal context.
Fix a typo