Update description of Mattermost Menu in issue_template.md
Update description of Mattermost Menu in issue_template.md

Update description of Mattermost Menu in issue_template.md.

Also update location of the desktop app version, given it's now under **Help**
yuya-oc/electron-mattermost,St-Ex/desktop,mattermost/desktop,yuya-oc/desktop,yuya-oc/electron-mattermost,mattermost/desktop,yuya-oc/desktop,jnugh/desktop,St-Ex/desktop,yuya-oc/desktop,jnugh/desktop,mattermost/desktop,yuya-oc/electron-mattermost,mattermost/desktop,mattermost/desktop,St-Ex/desktop,jnugh/desktop