mumble: Add option to set SuperUser password

Add an option in the Mumble app page to set SuperUser password. SuperUser account can be used to manage channel and group permissions in Mumble. One can access these options after logging in with username 'SuperUser' with super user password. See Murmur Guide. Currently FreedomBox does not allow for a way to set this password for Mumble. Users will need to manually retrieve or set the password. To fix the problem do the following:

• Move MumbleAppView to views.py under the plinth/modules/mumble directory. See ssh as example.
• Implement MumbleForm in forms.py and add a field to input the super user password. See ssh app as example. Label should be "Set SuperUser password". Description for the form field should say "Optional. Leave this field blank to keep the current password. SuperUser password can be used to manage permissions in Mumble.". Use this form in MumbleAppView.
• Implement code for successful submission of the form. Implement an action script in actions/mumble. This action should set the password for mumble super user. See Mumble wiki page on setting the password. The operation should be run as 'mumble-server' user. Password should not be passed on the command line. Instead send it on standard input using -reasupw option.

This issue is nominated for 'Contribute Invite' (ping @dannyhaidar-guest):

• Task Area: Coding
• Effort Rating: 3/5
• Skill Required: Python, Django Views/Templates/Forms