Add a shortcut to quickly link to channels

Description

Add the ability to quickly link to public channels using "!" followed by the channel handle.

Typing a word starting with ! should bring up an autocomplete menu containing a list of public channels.
(In the autocomplete, channel handle should be bolded, followed by the channel name as help text.)

When posted, this should create a link to the selected channel.

For example, !town-square should link to Town Square.

Related discussions:
https://forum.mattermost.org/t/referencing-channels/762
https://github.com/mattermost/platform/issues/2203

QA Test Steps

None

Status

Mana

8

Assignee

Joram Wilander

QA Assignee

None

Reporter

Lindsay Brock

Epic Link

None

Fix versions

None

Mattermost Team

None

Sprint

None

QA Testing Areas

None

GitHub Issue

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Components

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Configure