Text color of event types and difficulty levels

Changing the text color of event types and difficulty levels taking the background color into account. Similar to what it is done with tracks. See issue https://github.com/openSUSE/osem/issues/1033

Closes https://github.com/openSUSE/osem/issues/1078

app/views/admin/difficulty_levels/index.html.haml

@@ -20,7 +20,7 @@
             %td
               = difficulty_level.description
             %td
-              %span.label{style: "background-color: #{difficulty_level.color};"}
+              %span.label{style: "background-color: #{difficulty_level.color}; color: #{ contrast_color(difficulty_level.color) };"}
                 = difficulty_level.color
             %td
               .btn-group{role: "group"}

app/views/admin/event_types/index.html.haml

@@ -28,7 +28,7 @@
               = "#{event_type.minimum_abstract_length} - #{event_type.maximum_abstract_length}"
               Words
             %td
-              %span.label{style: "background-color: #{event_type.color};"}
+              %span.label{style: "background-color: #{event_type.color}; color: #{ contrast_color(event_type.color) };"}
                 = event_type.color
             %td
               .btn-group{role: "group"}
@@ -39,4 +39,3 @@
 .row
   .col-md-12.text-right
     = link_to 'Add Event Type', new_admin_conference_program_event_type_path(@conference.short_title), class: 'btn btn-primary'
-

app/views/proposal/show.html.haml

@@ -76,7 +76,7 @@
             %dt Difficulty:
             %dd
               - if @event.difficulty_level
-                %span.label{:style =>"background-color: #{@event.difficulty_level.color};"}
+                %span.label{:style =>"background-color: #{@event.difficulty_level.color}; color: #{ contrast_color(@event.difficulty_level.color) };"}
                   = @event.difficulty_level.title
 
           - if @event.require_registration