gui, lib/config, lib/model: Allow absolute values for minimum disk free space (fixes #3307)

This deprecates the current minDiskFreePct setting and introduces
minDiskFree. The latter is, in it's serialized form, a string with a
unit. We accept percentages ("2.35%") and absolute values ("250 k", "12.5
Gi"). Common suffixes are understood. The config editor lets the user
enter the string, and validates it.

We still default to "1 %", but the user can change that to an absolute
value at will.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4087
LGTM: AudriusButkevicius, imsodin

gui/default/syncthing/settings/settingsModalView.html

@@ -78,6 +78,25 @@
             <label translate for="GlobalAnnServersStr">Global Discovery Servers</label>
             <input ng-disabled="!tmpOptions.globalAnnounceEnabled" id="GlobalAnnServersStr" class="form-control" type="text" ng-model="tmpOptions._globalAnnounceServersStr">
           </div>
+
+          <div class="form-horizontal">
+            <div class="form-group" ng-class="{'has-error': settingsEditor.minHomeDiskFree.$invalid && settingsEditor.minHomeDiskFree.$dirty}">
+              <label class="col-xs-12" for="minHomeDiskFree"><span translate>Minimum Free Disk Space</span></label><br/>
+              <div class="col-xs-9"><input name="minHomeDiskFree" id="minHomeDiskFree" class="form-control" type="number" ng-model="tmpOptions.minHomeDiskFree.value" required min="0" step="0.01"></div>
+              <div class="col-xs-3"><select class="col-sm-3 form-control" ng-model="tmpOptions.minHomeDiskFree.unit">
+                <option value="%">%</option>
+                <option value="kB">kB</option>
+                <option value="MB">MB</option>
+                <option value="GB">GB</option>
+                <option value="TB">TB</option>
+              </select></div>
+              <p class="col-xs-12 help-block">
+                <span translate ng-show="settingsEditor.minHomeDiskFree.$invalid">Enter a non-negative number (e.g., "2.35") and select a unit. Percentages are as part of the total disk size.</span>
+                <span translate ng-hide="settingsEditor.minHomeDiskFree.$invalid">This setting controls the free space required on the home (i.e., index database) disk.</span>
+              </p>
+            </div>
+          </div>
+
         </div>
 
         <div class="col-md-6">