Add an option to set a default logo to be placed for all created meetings.

The default logo is inserted as a create meeting param for all meetings that
do not have a `logo` param defined. Two new properties were included at
bigbluebutton-web configuration:

- [Boolean] `useDefaultLogo`: enables/disables the use of the default logo
  disabled as default

- [String] `defaultLogoURL`: The default logo URL to be fetched by the client
  bigbluebutton logo as default