Support displaying time in absolute format

This adds two config items: useAbsoluteTimeFormat (boolean) and
absoluteTimeFormatMinAge (string, number + unit ('d'|'h'|'m'|'s')).

When `useAbsoluteTimeFormat` is true, the Timeago component will display
absolute time if the time is at least `absoluteTimeFormatMinAge`
from now. If `longFormat` prop is true, the fully formatted time
is displayed. Otherwise, the format is determined by the `time` prop:
(1) if `time` is on the same day of now, display hour and minute;
(2) if `time` is in the same month of now, display day and hour;
(3) if `time` is in the same year of now, display month and day;
(4) otherwise, display year and month.

If it should display relative time, the format is the same as before.

src/components/settings_modal/tabs/general_tab.vue

@@ -217,6 +217,29 @@
             {{ $t('settings.no_rich_text_description') }}
           </BooleanSetting>
         </li>
+        <li>
+          <BooleanSetting
+            path="useAbsoluteTimeFormat"
+            expert="1"
+          >
+            {{ $t('settings.absolute_time_format') }}
+          </BooleanSetting>
+        </li>
+        <ul
+          class="setting-list suboptions"
+          v-if="mergedConfig.useAbsoluteTimeFormat"
+        >
+          <li>
+            <UnitSetting
+              path="absoluteTimeFormatMinAge"
+              unit-set="time"
+              :units="['s', 'm', 'h', 'd']"
+              :min="0"
+            >
+              {{ $t('settings.absolute_time_format_min_age') }}
+            </UnitSetting>
+          </li>
+        </ul>
         <h3>{{ $t('settings.attachments') }}</h3>
         <li>
           <BooleanSetting