Text trigger should trigger on text in slack attachments.

Description

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QA Test Steps

I don't know if we need to manually test this, the unit tests are pretty thorough. But if we do, you can use a curl to post a message with a slack attachment. Here's what I used for the demo:
```
curl -i -X POST -H 'Authorization: Bearer d13eoojznid88n61ziuxaej95o' -d '{"channel_id":"kiof53je3fyrjeb4ucnrmnyh8a", "message":"Test message #testing", "props":{"attachments": [{"pretext": "This is a notification from your security system.","text": "Er meh gerd, everything is on :fire:. Send help now. #sec"}]}}' http://localhost:8065/api/v4/posts
```

Mana

None

Assignee

Christopher Poile

QA Assignee

Dylan Haussermann

Reporter

Christopher Speller

Epic Link

None

Fix versions

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Mattermost Team

Workflows

Sprint

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Labels

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QA Testing Areas

Plugins

GitHub Issue

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Components

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