api: support the expires_in param on all GET requests.

`expires_in` is an alternative to `expiry` that supports durations other
than days:

* https://danbooru.donmai.us/posts.json?expires_in=30s
* https://danbooru.donmai.us/posts.json?expires_in=5min
* https://danbooru.donmai.us/posts.json?expires_in=1hour
* https://danbooru.donmai.us/posts.json?expires_in=1week
* https://danbooru.donmai.us/posts.json?expires_in=1month

app/logical/duration_parser.rb

@@ -0,0 +1,27 @@
+module DurationParser
+  def self.parse(string)
+    string =~ /(\d+)(s(econds?)?|mi(nutes?)?|h(ours?)?|d(ays?)?|w(eeks?)?|mo(nths?)?|y(ears?)?)?/i
+
+    size = $1.to_i
+    unit = $2
+
+    case unit
+    when /^s/i
+      size.seconds
+    when /^mi/i
+      size.minutes
+    when /^h/i
+      size.hours
+    when /^d/i
+      size.days
+    when /^w/i
+      size.weeks
+    when /^mo/i
+      size.months
+    when /^y/i
+      size.years
+    else
+      size.seconds
+    end
+  end
+end