Add Snackbar and Toolbar Button to Forum List

This MR is a small change that replaces the custom UI elements in
the forum list with a Snackbar and a Toolbar menu.

It also fixes a background color that was still present with fragment from the NavDrawer.

Before:

![before](/uploads/6dcc545827c035dcffcdf7863893753e/before.png)

After:

![after](/uploads/427de01d8bec779ee81e6ebded3fcd2b/after.png)

It addresses one part (2b) of #305 and is part of #121.


See merge request !163