MDL-45930 filemanager: Don't display filesize when unknown

Thanks to Marina Glancy for the patch

Conflicts:
	theme/bootstrapbase/style/moodle.css

theme/base/style/filemanager.css

@@ -253,6 +253,7 @@ a.ygtvspacer:hover {color:transparent;text-decoration:none;}
 .file-picker.fp-select.loading .fp-select-loading {display:block;}
 .file-picker.fp-select.loading form {display:none;}
 .fp-select .fp-dimensions.fp-unknown {display:none;}
+.fp-select .fp-size.fp-unknown {display:none;}
 
  /*
  * File Manager

theme/bootstrapbase/less/moodle/filemanager.less

@@ -760,6 +760,9 @@ a.ygtvspacer:hover {
 .fp-select .fp-dimensions.fp-unknown {
     display: none;
 }
+.fp-select .fp-size.fp-unknown {
+    display: none;
+}
 // File Manager
 .filemanager-loading{
     display: none;