Inherit text color instead of defining it again

This way it's easier to customize colors, particularly when links use
the same colors as the text surrounding them.

Note that the `markdown-editor-header` isn't displayed since commit
76b7f66fb, which was probably an unintended side-effect. So we're
modifying the colors here as well instead of removing the element; we
might display it again in the future.

Also note the change in `.supported` only affects the proposals section;
budget investments use a different color for the `.supported` sucess
message. Not sure whether this was originally intended or done by
accident.

app/assets/stylesheets/admin.scss

@@ -135,7 +135,7 @@ $table-header:   #ecf1f6;
   }
 
   .title-bar {
-    color: #000;
+    color: inherit;
     position: absolute;
     right: 12px;
   }
@@ -840,6 +840,7 @@ table {
 .fullscreen {
 
   .fullscreen-container {
+    color: $white;
 
     a {
       line-height: 3rem;
@@ -852,7 +853,6 @@ table {
     .markdown-editor-header {
       vertical-align: top;
       display: inline-block;
-      color: $white;
 
       @include breakpoint(medium) {
         line-height: 3rem;
@@ -891,7 +891,7 @@ table {
     }
 
     a {
-      color: $white;
+      color: inherit;
     }
   }
 }

app/assets/stylesheets/layout.scss

@@ -1490,7 +1490,7 @@ table {
 .button.button-facebook,
 .button.button-google,
 .button.button-wordpress {
-  color: $text;
+  color: inherit;
   font-weight: bold;
 
   &::before {

app/assets/stylesheets/layout/locale_switcher.scss

@@ -20,7 +20,7 @@
     }
 
     label {
-      color: #fff;
+      color: inherit;
       font-size: $tiny-font-size;
       font-weight: normal;
     }

app/assets/stylesheets/participation.scss

@@ -81,7 +81,7 @@
   }
 
   .supported {
-    color: $text;
+    color: inherit;
     margin-top: rem-calc(12);
   }
 }