delegates submit button visibility to stylesheet

Instead of deciding in the template to only show the submit button for
the text input action on mobile, we make that determination in the base
stylesheet. This allows greater flexibility in where and how this submit
button can be displayed.

src/botui.html

@@ -25,7 +25,7 @@
           <input type="text" ref="input" :type="action.text.sub_type"
           v-model="action.text.value" class="botui-actions-text-input" :placeholder="action.text.placeholder"
           :size="action.text.size" :value="action.text.value" :class="action.text.cssClass" required v-focus/>
-          <button v-if="isMobile" class="botui-actions-text-submit">Go</button>
+          <button type="submit" class="botui-actions-text-submit">Go</button>
         </form>
         <div v-if="action.type == 'button'" class="botui-actions-buttons" :class="action.cssClass">
           <button type="button" :class="button.cssClass"

src/styles/botui.scss

@@ -35,7 +35,7 @@
   &.human {
     float: right;
   }
-  
+
   iframe {
     width: 100%;
   }
@@ -57,3 +57,9 @@
     margin-left: 10px;
   }
 }
+
+.botui-actions-text-submit {
+  @media (min-width: 400px) {
+    display: none;
+  }
+}