Add support of the multi-word text arguments without quotes

[iomodo]

To input multi-word text argument in the slash command autocomplete we use quotes. Here is the code how we parse it.
But if model.AutocompleteArgTypeText is the last argument of a command tree there is no need to have quotes for a multi-word input.
Change the code to support multi-word input without quotes for the last argument of the command.

Example: /agenda queue "message with quotes" vs /agenda queue message without quotes

Changes should be done only on a server.

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JIRA: https://mattermost.atlassian.net/browse/MM-25545

[mukulrawat1986]

Hi, I would like to work on this.

[reflog]

@mukulrawat1986 - are you still working on this or can we reassign? cc @iomodo

[mukulrawat1986]

Yea, I am still working on this.

[Shubhamchinda]

I'd like to take this.

[mukulrawat1986]

@reflog Please reassign this to @Shubhamchinda

[jfrerich]

This issue appears stale and I’m adding the Up For Grabs label. If work is ongoing, please remove the label. If there is an existing PR, please also link it to the issue. Thanks!

[Mavintyre]

Still learning Go, but I'd like to take a swing at this.

[Mavintyre]

Read all the documentation for contributions, but if I missed anything, let me know.