Use has-fa-icon in agree/disagree comment buttons

Note we're using the in-favor HTML class instead of the in_favor class
so we're consistent with our conventions for HTML classes and because we
use the in-favor class in similar places.

Also note the styles of the legislation process annotations/comments
buttons is now similar to the styles in the other sections. Previously,
the colors didn't have enough contrast and there was a very strange
margin between the "thumbs up" icon and the number of people agreeing
(that margin wasn't present between the "thumbs down" icon and the
number of people disagreeing).

spec/system/comments/proposals_spec.rb

@@ -441,7 +441,7 @@ describe "Commenting proposals" do
       visit proposal_path(proposal)
 
       within("#comment_#{comment.id}_votes") do
-        within(".in_favor") do
+        within(".in-favor") do
           expect(page).to have_content "1"
         end
 
@@ -459,7 +459,7 @@ describe "Commenting proposals" do
       within("#comment_#{comment.id}_votes") do
         click_button "I agree"
 
-        within(".in_favor") do
+        within(".in-favor") do
           expect(page).to have_content "1"
         end
 
@@ -477,13 +477,13 @@ describe "Commenting proposals" do
       within("#comment_#{comment.id}_votes") do
         click_button "I agree"
 
-        within(".in_favor") do
+        within(".in-favor") do
           expect(page).to have_content "1"
         end
 
         click_button "I disagree"
 
-        within(".in_favor") do
+        within(".in-favor") do
           expect(page).to have_content "0"
         end
 
@@ -501,13 +501,13 @@ describe "Commenting proposals" do
       within("#comment_#{comment.id}_votes") do
         click_button "I agree"
 
-        within(".in_favor") do
+        within(".in-favor") do
           expect(page).to have_content "1"
         end
 
         click_button "I agree"
 
-        within(".in_favor") do
+        within(".in-favor") do
           expect(page).to have_content "1"
         end