Use HTML beautifier to indent ERB files

We had inconsistent indentation in many places. Now we're fixing them
and adding a linter to our CI so we don't accidentally introduce
inconsistent indentations again.

app/views/proposals/retire_form.html.erb

@@ -7,7 +7,7 @@
     <h3><%= link_to @proposal.title, @proposal %></h3>
 
     <div data-alert class="callout warning">
-     <%= t("proposals.retire_form.warning") %>
+      <%= t("proposals.retire_form.warning") %>
     </div>
 
     <%= render "shared/globalize_locales", resource: @proposal, manage_languages: false %>

app/views/proposals/share.html.erb

@@ -1,9 +1,9 @@
 <% provide :title, @proposal.title %>
 <% provide :social_media_meta_tags do %>
-<%= render "shared/social_media_meta_tags",
-           social_url: proposal_url(@proposal),
-           social_title: @proposal.title,
-           social_description: @proposal.summary %>
+  <%= render "shared/social_media_meta_tags",
+             social_url: proposal_url(@proposal),
+             social_title: @proposal.title,
+             social_description: @proposal.summary %>
 <% end %>
 <% content_for :canonical do %>
   <%= render "shared/canonical", href: proposal_url(@proposal) %>