Confirm before sending (at)all, (at)channel message

Description

Opening ticket based on volunteer contributor. https://mattermost.uservoice.com/forums/306457-general/suggestions/18607111-confirm-with-ping-warning-before-sending-all-eve

Propose when sending a message containing (at) all or (at) channel create a pop-up dialog with confirmation:

TITLE:

Confirm sending notifications to entire channel

TEXT:

```
By using @all or @channel you are about to send notifications to XXX people. Are you want to do this?
```

BUTTONS

Cancel
Confirm

QA Test Steps

None

Status

Mana

None

Assignee

Jason Blais

QA Assignee

None

Reporter

Ian Tien

Epic Link

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Fix versions

None

Mattermost Team

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Sprint

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Labels

QA Testing Areas

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GitHub Issue

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Components

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Configure