client/general: add empty href for link buttons

In e464e69 I removed href='#' but I noticed that it broke some things.
Readding href serves two purposes:

- it makes links reachable with Tab key
- it makes links clickable with Enter key

The alternative to this approach was to introduce [tabindex] and [role]
attributes. But not only using tabindex=0 with <a/> is questionable,
it'd require adding a keyboard handler that'd intercept space and return
key presses and simulated link clicks. Since it's best to leave this
kind of thing to the native UI, I went with readding hrefs instead. I
believe that hash hrefs, even though being a common practice, are silly,
so I decided to settle down with empty hrefs.

As a bonus, I added a snippet that prevents middle mouse clicks from
opening such links/buttons in new tabs, which was the motivation for
e464e69.

client/js/util/views.js

@@ -476,6 +476,14 @@ document.addEventListener('input', e => {
     }
 });
 
+// prevent opening buttons in new tabs
+document.addEventListener('click', e => {
+    if (e.target.getAttribute('href') === '' && e.which === 2) {
+        console.log('prevented');
+        e.preventDefault();
+    }
+});
+
 module.exports = misc.arrayToObject([
     htmlToDom,
     getTemplate,