MDL-56910 filepicker: fix eranously removed styles

No idea what happened to my testing of this change, but it was wrong.

theme/boost/scss/moodle/filemanager.scss

@@ -603,6 +603,10 @@ a.ygtvspacer:hover {
     background-color: #ebebe4;
 }
 
+.fitem.disabled .fp-btn-choose {
+    @extend .text-muted;
+}
+
 .fitem.disabled .filepicker-filelist .filepicker-filename {
     display: none;
 }

theme/bootstrapbase/less/moodle/filemanager.less

@@ -800,6 +800,9 @@ a.ygtvspacer:hover {
 .fitem.disabled .filemanager-container {
     background-color: #ebebe4;
 }
+.fitem.disabled .fp-btn-choose {
+    .muted
+}
 .fitem.disabled .filepicker-filelist .filepicker-filename {
     display: none;
 }

theme/bootstrapbase/style/moodle.css

@@ -5497,6 +5497,9 @@ a.ygtvspacer:hover {
 .fitem.disabled .filemanager-container {
   background-color: #ebebe4;
 }
+.fitem.disabled .fp-btn-choose {
+  color: #999;
+}
 .fitem.disabled .filepicker-filelist .filepicker-filename {
   display: none;
 }