Move Clone New into repo dropdown, remove sidebar links

Remove the Repositories and Clone New links from the sidebar.
Add a 'Clone New...' option to the repo dropdown that shows the
clone form in the content frame when selected.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/resources/templates/nav.html

@@ -10,16 +10,12 @@
 <select name="repo">
 <option value="">-- Select repo --</option>
 <option th:each="r : ${repositories}" th:value="${r}" th:text="${r}" th:selected="${r == selectedRepo}"></option>
+<option value="__clone__">Clone New...</option>
 </select>
 <br>
 <input type="submit" value="Go">
 </form>
 
-<hr>
-
-<a href="/repos" target="content">Repositories</a><br>
-<a href="/clone-form" target="content">Clone New</a><br>
-
 <th:block th:if="${selectedRepo != null}">
 <hr>
 <b th:text="${selectedRepo}"></b><br>