Desktop: Replace at-mention DOM scraping with proper webapp<->desktop communication using existing communication bridge

Description

We need to replace the Desktop app's unread/at-mention DOM scraping code with a more direct and reliable means of getting updates from the webapp for unread and at-mention counts.

This will have server/webapp version implications, so need to ensure that we remain backwards compatible!

QA Test Steps

1. Load a server that has a number of channels with a mix of unreads and mentions in the Desktop App.
2. Ensure that the tab includes a mention badge totalling the number of mentions in the loaded webapp
3. Manually view all channels with at-mentions but leave channels with unreads as unread
4. Ensure that the tab indicates there are unreads

Mana

6

Assignee

Unassigned

QA Assignee

None

Reporter

Dean Whillier

Epic Link

None

Fix versions

Mattermost Team

Apps

Sprint

None

Labels

QA Testing Areas

Mentions

GitHub Issue

None

Components

None
Configure