Prevent negative values for number inputs

- settings: incoming/outgoing rate limit - min: 0
- folder: maximum age (staggered file versioning) - min: 0
- help texts for validation

gui/syncthing/folder/editFolderModalView.html

@@ -118,10 +118,11 @@
                 <p class="help-block"><span translate>Files are moved to date stamped versions in a .stversions folder when replaced or deleted by Syncthing.</span> <span translate>Versions are automatically deleted if they are older than the maximum age or exceed the number of files allowed in an interval.</span></p>
                 <p translate class="help-block">The following intervals are used: for the first hour a version is kept every 30 seconds, for the first day a version is kept every hour, for the first 30 days a version is kept every day, until the maximum age a version is kept every week.</p>
                 <label translate for="staggeredMaxAge">Maximum Age</label>
-                <input name="staggeredMaxAge" id="staggeredMaxAge" class="form-control" type="number" ng-model="currentFolder.staggeredMaxAge" required></input>
+                <input name="staggeredMaxAge" id="staggeredMaxAge" class="form-control" type="number" ng-model="currentFolder.staggeredMaxAge" required min="0"></input>
                 <p class="help-block">
                   <span translate ng-if="folderEditor.staggeredMaxAge.$valid || folderEditor.staggeredMaxAge.$pristine">The maximum time to keep a version (in days, set to 0 to keep versions forever).</span>
                   <span translate ng-if="folderEditor.staggeredMaxAge.$error.required && folderEditor.staggeredMaxAge.$dirty">The maximum age must be a number and cannot be blank.</span>
+                  <span translate ng-if="folderEditor.staggeredMaxAge.$error.min && folderEditor.staggeredMaxAge.$dirty">A negative number of days doesn't make sense.</span>
                 </p>
               </div>
               <div class="form-group" ng-if="currentFolder.fileVersioningSelector == 'staggered'">