Improve version verbiage

The most common case is now cleaner:

    version: stable → nothing

The meaning of "latest" is now clearer:

    version: latest → development version

and a useless colon is dropped:

    version: X → version X

Versions are always always on their own (centered) line now underneath
the subproject name, instead of potentially sitting alongside them for
short names (like "CLI").

lib/nextstrain/sphinx/theme/layout.html

@@ -45,9 +45,15 @@
       {%- set nav_version = current_version %}
     {% endif %}
     {% if nav_version %}
-      <span>
-        version: {{ nav_version }}
-      </span>
+      <div class="version">
+        {% if nav_version == "stable" %}
+          {# don't show anything #}
+        {% elif nav_version == "latest" %}
+          development version
+        {% else %}
+          version {{ nav_version }}
+        {% endif %}
+      </div>
     {% endif %}
 
   </div>

lib/nextstrain/sphinx/theme/static/css/nextstrain.css

@@ -85,7 +85,7 @@ footer span.commit code,
   font-weight: 500;
   color: var(--heading-color);
 }
-.wy-side-nav-search > div.subproject > span { /* version name */
+.wy-side-nav-search > div.subproject > .version { /* version name */
   display: inline-block;
   font-size: 1.1rem;
   font-weight: 300;