enable --hidden and add an option in settings to start mnimized

Description

re-enable the `--hidden` flag so the app can start hidden if that's what the user prefers
add a nested option to start on login but hidden

make sure we are still able to receive notifications if used.

QA Test Steps

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Mana

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Assignee

Unassigned

QA Assignee

None

Reporter

Guillermo Vayá

Epic Link

None

Fix versions

Mattermost Team

Web Platform

Sprint

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Labels

QA Testing Areas

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GitHub Issue

None

Components

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