Added absolute dates to user list serializer

Added fields last_seen_at, last_emailed_at, and created_at to the admin user list serializer, to allow client applications to use those dates without having to parse the age string.

Referred to at: https://meta.discourse.org/t/api-returns-relative-times-instead-of-absolute-time-values
Exit if a non zero status code is returned.
Merge pull request #3056 from tgxworld/exit_on_non_zero_status_code

Exit if a non zero status code is returned.
Merge pull request #3055 from techAPJ/patch-3

FEATURE: download user posts archive
Merge pull request #3051 from RauchF/api_absolute_days

Added absolute dates to user list serializer
copy improvements on data export
add + to keyboard help key simul combos
remove a bunch of needless css prefixes
who the hell puts a } in front of a SASS line?
add back in a few -webkit- css3 prefixes

due to http://shouldiprefix.com/ and iOS mainly!
add back in @-webkit-keyframes
clean up CSS3 animations a bit
FEATURE: default emoji override
FIX: 1.9 regression on i18n strings with variables
FIX: proper redirection when deleting a user
Fix typo in Gemfile

[ci skip]
FIX: cache emojis for 1 year
Rename CsvExportLog to UserExport
Ruby 2.2 fixes
PERF: improve rendering speed of topic header

This also ensures we have a clean point to override rendering of topic list header
in case we decide to add/remove columns, you no longer need to override the
entire discovery template, only the header and item templates.