Web: Private channels do not appear in channel link autocomplete

Description

Environment: Firefox/Mac on community-daily

Repro steps:
1) Create a private channel named `privatechannel`
2) In the post textbox, type `~priv`

Observed: No autocomplete entry appears for `~privatechannel` or any other private channel
Expected: An autocomplete entry appears for `~privatechannel`. If the current user is a member of the linked channel, then show it as a link when posted, if not it should appear as normal text (as to not give away that the channel exists)

QA Test Steps

See description

Reporter

None

Status

Mana

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Assignee

Unassigned

QA Assignee

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Reporter

Harrison Healey

Epic Link

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Fix versions

Mattermost Team

Platform

Sprint

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