Add term name to tools variable substitutions

test plan:
* add a new custom field, `term_name=$Canvas.term.name`, to an external tool
* open launch link to external tool
* verify that `custom_term_name` was part of the launch request and is correct

doc/api/tools_variable_substitutions.md

@@ -480,6 +480,15 @@ returns the current course's term start date.
 ```
 YYY-MM-DD HH:MM:SS -0700
 ```
+## Canvas.term.name
+returns the current course's term name.
+
+**Availability**: **  
+**Launch Parameter**: *canvas_term_name*  
+
+```
+W1 2017
+```
 ## CourseSection.sourcedId
 returns the current course sis source id
 to return the section source id use Canvas.course.sectionIds.

lib/lti/variable_expander.rb

@@ -52,6 +52,7 @@ module Lti
     COURSE_GUARD = -> { @context.is_a? Course }
     TERM_START_DATE_GUARD = -> { @context.is_a?(Course) && @context.enrollment_term &&
                                  @context.enrollment_term.start_at }
+    TERM_NAME_GUARD = -> { @context.is_a?(Course) && @context.enrollment_term&.name }
     USER_GUARD = -> { @current_user }
     SIS_USER_GUARD = -> { @current_user && @current_user.pseudonym && @current_user.pseudonym.sis_user_id }
     PSEUDONYM_GUARD = -> { sis_pseudonym }
@@ -516,6 +517,16 @@ module Lti
                        -> { @context.enrollment_term.start_at },
                        TERM_START_DATE_GUARD
 
+    # returns the current course's term name.
+    # @example
+    #   ```
+    #   W1 2017
+    #   ```
+    register_expansion 'Canvas.term.name', [],
+                        -> { @context.enrollment_term.name },
+                        TERM_NAME_GUARD,
+                        default_name: 'canvas_term_name'
+
     # returns the current course sis source id
     # to return the section source id use Canvas.course.sectionIds
     # @launch_parameter lis_course_section_sourcedid

spec/lib/lti/capabilities_helper_spec.rb

@@ -74,6 +74,7 @@ module Lti
     let(:valid_enabled_caps){ %w(ToolConsumerInstance.guid Membership.role CourseSection.sourcedId) }
     let(:supported_capabilities){
       %w(ToolConsumerInstance.guid
+         Canvas.term.name
          CourseSection.sourcedId
          Membership.role
          Person.email.primary

spec/lib/lti/variable_expander_spec.rb

@@ -753,6 +753,27 @@ module Lti
           expect(exp_hash[:test]).to eq '2015-05-21 17:01:36'
         end
 
+        it 'has a functioning guard for $Canvas.term.name when term.name is not set' do
+          term = course.enrollment_term
+          exp_hash = {test: '$Canvas.term.name'}
+          variable_expander.expand_variables!(exp_hash)
+
+          unless term && term.name
+            expect(exp_hash[:test]).to eq '$Canvas.term.name'
+          end
+        end
+
+        it 'has substitution for $Canvas.term.name when term.name is set' do
+          course.enrollment_term ||= EnrollmentTerm.new
+          term = course.enrollment_term
+
+          term.name = 'W1 2017'
+          term.save
+          exp_hash = {test: '$Canvas.term.name'}
+          variable_expander.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq 'W1 2017'
+        end
+
         it 'has substitution for $Canvas.externalTool.url' do
           course.save!
           tool = course.context_external_tools.create!(:domain => 'example.com', :consumer_key => '12345', :shared_secret => 'secret', :privacy_level => 'anonymous', :name => 'tool')