Mattermost-server: Allow users to specify different desktop notification sounds per channel

[mattermod]

When a user selects "Notification Preferences > Send Desktop Notifications", for a specific channel, we should add an option to this menu for "Notification Sound"

Mock-up:

Functionality:

• The channel notification preference should initially follow the global account setting notification preference
• If user changes from the global defaults, a ‘reset to defaults’ button appears in the top right of the section. Clicking this will return the settings back to the global defaults
• If the channel notification preference is changed here, it is then decoupled from the global setting.
• If the user edits the channel preferences notification sound, it should no longer be linked to the notification sound selected in account settings.
• If “For all activity” or “Only for mentions” is selected, a new section is added to the channel notifications settings to choose the notification sound.
• If "Never" is selected, then hide the settings for "notification sound

Note: these settings already implemented at the Account Settings level (see Account Settings > Notifications > Desktop Notifications > Notification Sound, related ticket that was completed )), but this is introducing the same control at the channel level to specify specific sounds per channel.

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JIRA: https://mattermost.atlassian.net/browse/MM-30432

[hmhealey]

Tip for implementing this: the setting's value will likely be stored in the NotifyProps field of the ChannelMember object corresponding to the current user and channel.

[marcelo-cardozo]

I would like to work in this one :)