Ensure consistent appearance of btn-link

After migrating `link_to` to `button_to` (in commit d4ecdb5), we need to adjust the styles for a uniform and correct appearance.

Amends d4ecdb5

Author: MrSerth

app/assets/stylesheets/base.css.scss

@@ -181,13 +181,16 @@ table.vertical-table {
     background-color: #eeeeee;
   }
 
-  a {
+  a,
+  .btn-link {
     padding: 10px 60px;
     background-color: #f5f5f5;
     display: inline-block;
   }
   a:hover,
-  a:focus {
+  a:focus,
+  .btn-link:hover,
+  .btn-link:focus {
     background-color: #f0f0f0;
     color: black;
     text-decoration: none;

app/assets/stylesheets/bootstrap_and_overrides.css.scss

@@ -196,3 +196,7 @@ a.no-border:visited {
     border-radius: 2px;
   }
 }
+
+.btn-link {
+  font-size: 14px;
+}

app/assets/stylesheets/scaffolds.css.scss

@@ -30,7 +30,8 @@ blockquote {
   padding: 0 16px;
 }
 
-a {
+a,
+.btn-link {
   color: #333;
 
   &:hover {

app/views/users/shared/_links.html.slim

@@ -22,5 +22,6 @@
   - if devise_mapping.omniauthable?
     - resource_class.omniauth_providers.each do |provider|
       = button_to(t('devise.shared.links.sign_in_with_provider', provider: OmniAuth::Utils.camelize(provider)),
-              omniauth_authorize_path(resource_name, provider), method: :post)
-      br
+              omniauth_authorize_path(resource_name, provider),
+              class: 'btn btn-link p-0 border-0',
+              method: :post)