Update settings text for "Flash taskbar icon when..."

[jasonblais]

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Summary
Update settings text for "Flash taskbar icon when..." to "Flash app window and taskbar icon when a new message is received", as the latter is the actual behaviour of the setting.

Issue link
#447

Test Cases

1. Go to File > Settings
2. Confirm the settings text reads Flash app window and taskbar icon when a new message is received
3. Confirm the help text reads If enabled, app window and taskbar icon flashes for a few seconds when a new message is received.

[jasonblais]

@yuya-oc I updated the help text based on your reply to #447.

Maybe have your help verify, and we can update for 3.6?

Artifact here: https://circleci.com/gh/mattermost/desktop/896#artifacts

[jasonblais]

Screenshot here from Windows 10:

[yuya-oc]

Thanks!

[caspChristian]

Is there any way to have Mattermost keep flashing until it gets focus? - as it is now I, and many in my team, keeps missing important messages because they where busy with something else for few minutes