Fixed giving page overlap. fixes #2834

Usernames longer than 15 characters are cut and '...' is appended at the end
Merge pull request #2836 from gratipay/fix-giving-page-overlap

Fixed giving page overlap. fixes #2834
git mv www/assets/%version/* www/assets/
put all the platform logos in the same directory
rewrite the static cache hooks

use etags instead of version and datetimes
create helper function to compute asset URLs with etags appended
replace uses of `asset_url` with calls to the new `asset()` helper
compile "dynamic" assets during wireup
add etags to URLs within CSS