aliases/implications: remove processing state.

Remove the `processing` state from aliases and implications. This state
was used to mark when an alias or implication had been approved but the
alias or implication was still being processed. Aliases in the
processing state were still considered active, so there was no
functional difference between the active state and the processing state.

This fixes a problem where it was possible for implications to get stuck
in the processing state. This happened when a BUR contained a duplicate
implication. Transitioning from the processing state to the active state
failed in this case because we used `update` instead of `update!`, which
meant validation errors were silently ignored.

test/unit/tag_alias_test.rb

@@ -27,7 +27,6 @@ class TagAliasTest < ActiveSupport::TestCase
       should allow_value('active').for(:status)
       should allow_value('deleted').for(:status)
       should allow_value('pending').for(:status)
-      should allow_value('processing').for(:status)
       should allow_value('error: derp').for(:status)
 
       should_not allow_value('ACTIVE').for(:status)

test/unit/tag_implication_test.rb

@@ -23,7 +23,6 @@ class TagImplicationTest < ActiveSupport::TestCase
       should allow_value('active').for(:status)
       should allow_value('deleted').for(:status)
       should allow_value('pending').for(:status)
-      should allow_value('processing').for(:status)
       should allow_value('error: derp').for(:status)
 
       should_not allow_value('ACTIVE').for(:status)