gitweb: cache age_epoch instead of age

gitweb: cache age_epoch instead of age

Caching the age (e.g. 10 minutes ago) instead of the age
epoch (e.g. 2015-04-20 11:16) prevents us from caching
the information properly.

Instead remove 'age' and 'age_string' from the project
info (and hence from the cached project info) and replace
them with 'age_epoch' which is not sensitive to the
current time.

A side effect of this is that the age and age_string
values are computed when the project list is actually
shown.  In addition since a single base time is used
when doing this two projects that have exactly the
same age will always show that even if it takes some
indeterminate time between when the first age is
computed and the second.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>