user controller index: consistently use uri->segment(5) to retrieve $offset

to match the way the parameter is passed to the pagination library
(no behaviour change).
users module: simplify filter, fix #727 (can't use pagination with filter)

CI pagination library doesn't link to urls with query strings.  But we were
using query strings to set filter parameters :(.

Simplify filter down to a single method parameter (i.e. a uri segment)
instead.  This prevents combining first_letter filtering with another
filter.  I think it's ok because the other filters will normally have quite
a small subset of all users.

So render_filter_first_letter() needed adapting away from query strings.
The new version would have more parameters but be much shorter...
out-of-tree uses would have to change significantly anwyay... and they'd
be suffering from the same problem we had here.  I've just coded it inline
for now, and dropped the old function.  I don't think it'll hurt for it
to be copy+pasted a bit.
Merge branch 'develop' of github.com:ci-bonfire/Bonfire into develop