Fine tune more downdrop settings, use SVG for labels, improve Repo Topic Edit form (#23626)

Although it seems that some different purposes are mixed in this PR,
however, they are all related, and can be tested together, so I put them
together to save everyone's time.

Diff: `+79 −84`, everything becomes much better.

### Improve the dropdown settings.

Move all fomantic-init related code into our `fomantic.js`

Fine-tune some dropdown global settings, see the comments.

Also help to fix the first problem in #23625 , cc: @yp05327 

The "language" menu has been simplified, and it works with small-height
window better.

### Use SVG instead of `<i class="delete icon">`

It's also done by `$.fn.dropdown.settings.templates.label` , cc:
@silverwind

### Remove incorrect `tabable` CSS class

It doesn't have CSS styles, and it was only in Vue. So it's totally
unnecessary, remove it by the way.

### Improve the Repo Topic Edit form

* Simplify the code
* Add a "Cancel" button
* Align elements

Before:

<details>

![image](https://user-images.githubusercontent.com/2114189/223325782-f09532de-0c38-4742-ba86-ed35cc9a858d.png)

</details>

After:

![image](https://user-images.githubusercontent.com/2114189/226796347-207feb0a-b3cd-4820-8a3e-01930bab1069.png)

web_src/css/base.css

@@ -1854,15 +1854,15 @@ footer .container .links > *:first-child {
 }
 
 footer .ui.language .menu {
-  max-height: 500px;
+  height: 500px;
+  max-height: calc(100vh - 60px);
   overflow-y: auto;
   margin-bottom: 7px;
 }
 
 footer .ui.language .svg {
   margin-right: 0.15em;
-  vertical-align: top;
-  margin-top: calc(2em - 16px);
+  margin-top: 1px;
 }
 
 footer .ui.left,
@@ -2387,6 +2387,10 @@ a.ui.label:hover {
   color: var(--color-text);
 }
 
+.ui.tertiary.button:focus {
+  color: var(--color-text-dark);
+}
+
 .ui.primary.label,
 .ui.primary.labels .label {
   background-color: var(--color-primary) !important;