Replace vue-multiselect with a custom component (#366)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/366
Co-authored-by: konrad <konrad@kola-entertainments.de>
Co-committed-by: konrad <konrad@kola-entertainments.de>

src/styles/theme/form.scss

@@ -128,10 +128,8 @@
   }
 }
 
-.field.has-addons {
-  .control .select select {
-    height: 2.25em;
-  }
+.field.has-addons .control .select select {
+    height: 100%;
 }
 
 .bigbuttons {

src/styles/theme/theme.scss

@@ -58,6 +58,10 @@ h1, h2, h3, h4, h5, h6 {
   border-radius: $radius;
 }
 
+.has-overflow {
+  overflow: visible;
+}
+
 .image.is-avatar {
   border-radius: 100%;
 }